Make the tests run on ruby-head

After modifying the Gemfile (as there's no listen release yet with
guard/listen#490), I updated the lockfile by
running `bundle update async-io simplecov-html listen` in Docker using
the `rubylang/ruby:master-nightly-bionic` image. I manually reverted the
change Bundler made to the `BUNDLED WITH` section of the lockfile (as we
probably don't want it to say 2.2.0.dev).

<Details><Summary>The <code>bundle update</code> log for the curious</Summary>

```
$ docker run --rm -it -v $(pwd):/app -w /app rubylang/ruby:master-nightly-bionic bash
root@0c8e1039460e:/app# bundle update async-io simplecov-html listen
Fetching https://github.com/guard/listen.git
Fetching gem metadata from https://rubygems.org/.........
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies......
Using rake 13.0.1
Using minitest 5.14.2
Fetching public_suffix 4.0.5
Fetching thread_safe 0.3.6
Fetching concurrent-ruby 1.1.7
Fetching zeitwerk 2.4.0
Installing zeitwerk 2.4.0
Installing thread_safe 0.3.6
Installing public_suffix 4.0.5
Installing concurrent-ruby 1.1.7
Fetching childprocess 4.0.0
Installing childprocess 4.0.0
Fetching contracts 0.16.0
Installing contracts 0.16.0
Fetching builder 3.2.4
Fetching middleware 0.1.0
Installing builder 3.2.4
Installing middleware 0.1.0
Fetching thor 1.0.1
Installing thor 1.0.1
Fetching cucumber-tag-expressions 2.0.4
Fetching ffi 1.13.1
Installing cucumber-tag-expressions 2.0.4
Fetching cucumber-cucumber-expressions 10.3.0
Installing cucumber-cucumber-expressions 10.3.0
Fetching multi_test 0.1.2
Fetching diff-lcs 1.4.4
Installing diff-lcs 1.4.4
Installing ffi 1.13.1 with native extensions
Fetching rspec-support 3.9.3
Installing multi_test 0.1.2
Installing rspec-support 3.9.3
Fetching ast 2.4.1
Installing ast 2.4.1
Fetching console 1.9.0
Fetching nio4r 2.5.4 (was 2.5.2)
Installing console 1.9.0
Installing nio4r 2.5.4 (was 2.5.2) with native extensions
Fetching timers 4.3.2 (was 4.3.0)
Installing timers 4.3.2 (was 4.3.0)
Using bundler 2.2.0.dev
Fetching debug_inspector 0.0.3
Fetching byebug 11.1.3
Installing debug_inspector 0.0.3 with native extensions
Installing byebug 11.1.3 with native extensions
Fetching mini_mime 1.0.2
Installing mini_mime 1.0.2
Fetching regexp_parser 1.8.0
Fetching mini_portile2 2.4.0
Fetching rack 2.2.3
Installing mini_portile2 2.4.0
Fetching coderay 1.1.3
Installing regexp_parser 1.8.0
Installing rack 2.2.3
Installing coderay 1.1.3
Fetching coffee-script-source 1.12.2
Installing coffee-script-source 1.12.2
Fetching execjs 2.7.0
Installing execjs 2.7.0
Fetching docile 1.3.2
Fetching dotenv 2.7.6
Installing dotenv 2.7.6
Installing docile 1.3.2
Fetching erubis 2.7.0
Fetching temple 0.8.2
Fetching fastimage 2.2.0
Fetching tilt 2.0.10
Installing tilt 2.0.10
Installing fastimage 2.2.0
Installing temple 0.8.2
Fetching hashie 3.6.0
Installing erubis 2.7.0
Installing hashie 3.6.0
Fetching interception 0.5
Installing interception 0.5 with native extensions
Using rexml 3.2.4
Fetching lazy_priority_queue 0.1.1
Installing lazy_priority_queue 0.1.1
Fetching liquid 4.0.3
Fetching rb-fsevent 0.10.4
Fetching libv8 8.4.255.0 (x86_64-linux)
Installing rb-fsevent 0.10.4
Installing liquid 4.0.3
Fetching memoist 0.16.2
Installing memoist 0.16.2
Fetching method_source 1.0.0
Installing method_source 1.0.0
Fetching padrino-support 0.14.4
Fetching oj 3.10.8
Installing padrino-support 0.14.4
Installing oj 3.10.8 with native extensions
Fetching parallel 1.19.2
Fetching stream 0.5.2
Installing parallel 1.19.2
Installing stream 0.5.2
Fetching servolux 0.13.0
Installing servolux 0.13.0
Fetching ruby2_keywords 0.0.2
Installing ruby2_keywords 0.0.2
Fetching redcarpet 3.5.0
Fetching rainbow 3.0.0
Installing rainbow 3.0.0
Installing redcarpet 3.5.0 with native extensions
Fetching ruby-progressbar 1.10.1
Installing ruby-progressbar 1.10.1
Installing libv8 8.4.255.0 (x86_64-linux)
Fetching simplecov-html 0.12.3 (was 0.12.2)
Fetching unicode-display_width 1.7.0
Fetching yard 0.9.25
Installing unicode-display_width 1.7.0
Fetching addressable 2.7.0
Installing simplecov-html 0.12.3 (was 0.12.2)
Installing addressable 2.7.0
Fetching tzinfo 1.2.7
Installing yard 0.9.25
Installing tzinfo 1.2.7
Fetching i18n 0.9.5
Installing i18n 0.9.5
Fetching hamster 3.0.0
Installing hamster 3.0.0
Using middleman-cli 5.0.0.rc.2 from source at `middleman-cli`
Fetching rspec-expectations 3.9.2
Installing rspec-expectations 3.9.2
Fetching parser 2.7.1.5
Installing parser 2.7.1.5
Fetching rspec-core 3.9.2
Installing rspec-core 3.9.2
Fetching rspec-mocks 3.9.1
Installing rspec-mocks 3.9.1
Fetching async 1.26.2
Fetching binding_of_caller 0.8.0
Installing async 1.26.2
Installing binding_of_caller 0.8.0 with native extensions
Fetching sys-uname 1.2.1
Fetching rb-inotify 0.10.1
Fetching sassc 2.4.0
Installing rb-inotify 0.10.1
Installing sys-uname 1.2.1
Installing sassc 2.4.0 with native extensions
Fetching nokogiri 1.10.10
Fetching rack-test 1.1.0
Fetching rack-protection 2.1.0
Installing rack-test 1.1.0
Installing rack-protection 2.1.0
Fetching coffee-script 2.4.1
Installing coffee-script 2.4.1
Fetching uglifier 4.2.0
Fetching haml 5.2.0
Installing haml 5.2.0
Installing uglifier 4.2.0
Installing nokogiri 1.10.10 with native extensions
Fetching kramdown 2.3.0
Fetching slim 4.1.0
Installing kramdown 2.3.0
Installing slim 4.1.0
Fetching mustermann 1.1.1
Fetching rgl 0.5.6
Fetching pry 0.13.1
Installing rgl 0.5.6
Installing mustermann 1.1.1
Installing pry 0.13.1
Fetching mini_racer 0.3.1
Installing mini_racer 0.3.1 with native extensions
Fetching simplecov 0.19.0
Installing simplecov 0.19.0
Fetching activesupport 6.0.3.3
Fetching rubocop-ast 0.5.0
Fetching padrino-helpers 0.14.4
Installing rubocop-ast 0.5.0
Installing padrino-helpers 0.14.4
Fetching rspec 3.9.0
Installing activesupport 6.0.3.3
Installing rspec 3.9.0
Fetching async-io 1.30.1 (was 1.30.0)
Installing async-io 1.30.1 (was 1.30.0)
Using listen 3.2.1 from https://github.com/guard/listen.git (at master@86e06ae)
Fetching xpath 3.2.0
Fetching sinatra 2.1.0
Installing xpath 3.2.0
Installing sinatra 2.1.0
Fetching pry-rescue 1.5.2
Installing pry-rescue 1.5.2
Fetching pry-stack_explorer 0.4.9.3
Installing pry-stack_explorer 0.4.9.3
Fetching async-dns 1.2.5
Fetching rubocop 0.92.0
Installing async-dns 1.2.5
Fetching capybara 3.33.0
Installing rubocop 0.92.0
Fetching protobuf-cucumber 3.10.8
Installing capybara 3.33.0
Using middleman-core 5.0.0.rc.2 from source at `middleman-core`
Fetching rubydns 2.0.2
Installing protobuf-cucumber 3.10.8
Installing rubydns 2.0.2
Fetching cucumber-messages 13.1.0
Installing cucumber-messages 13.1.0
Fetching cucumber-create-meta 2.0.2
Fetching cucumber-gherkin 15.0.2
Installing cucumber-create-meta 2.0.2
Installing cucumber-gherkin 15.0.2
Fetching cucumber-html-formatter 9.0.0
Fetching cucumber-core 8.0.1
Installing cucumber-html-formatter 9.0.0
Installing cucumber-core 8.0.1
Fetching cucumber-wire 4.0.1
Installing cucumber-wire 4.0.1
Fetching cucumber 5.1.2
Installing cucumber 5.1.2
Fetching rubocop-performance 1.8.1
Installing rubocop-performance 1.8.1
Fetching aruba 1.0.3
Installing aruba 1.0.3
Bundle updated!
```

</Details>

Gemfile

@@ -46,3 +46,6 @@ gem 'middleman-cli', path: 'middleman-cli'
 gem 'middleman-core', path: 'middleman-core'
 
 # gem 'middleman-sprockets', github: 'middleman/middleman-sprockets', require: false
+
+# Remove when there is a release with https://github.com/guard/listen/pull/490
+gem 'listen', github: 'guard/listen'

Gemfile.lock

@@ -1,3 +1,11 @@
+GIT
+  remote: https://github.com/guard/listen.git
+  revision: 86e06aeb0878dcd46fd635d620656748255d3f67
+  specs:
+    listen (3.2.1)
+      rb-fsevent (~> 0.10, >= 0.10.3)
+      rb-inotify (~> 0.9, >= 0.9.10)
+
 PATH
   remote: middleman-cli
   specs:
@@ -56,7 +64,7 @@ GEM
       timers (~> 4.1)
     async-dns (1.2.5)
       async-io (~> 1.15)
-    async-io (1.30.0)
+    async-io (1.30.1)
       async (~> 1.14)
     binding_of_caller (0.8.0)
       debug_inspector (>= 0.0.1)
@@ -118,6 +126,7 @@ GEM
     execjs (2.7.0)
     fastimage (2.2.0)
     ffi (1.13.1)
+    ffi (1.13.1-java)
     haml (5.2.0)
       temple (>= 0.8.0)
       tilt
@@ -132,9 +141,6 @@ GEM
     lazy_priority_queue (0.1.1)
     libv8 (8.4.255.0)
     liquid (4.0.3)
-    listen (3.2.1)
-      rb-fsevent (~> 0.10, >= 0.10.3)
-      rb-inotify (~> 0.9, >= 0.9.10)
     memoist (0.16.2)
     method_source (1.0.0)
     middleware (0.1.0)
@@ -146,9 +152,11 @@ GEM
     multi_test (0.1.2)
     mustermann (1.1.1)
       ruby2_keywords (~> 0.0.1)
-    nio4r (2.5.2)
+    nio4r (2.5.4)
+    nio4r (2.5.4-java)
     nokogiri (1.10.10)
       mini_portile2 (~> 2.4.0)
+    nokogiri (1.10.10-java)
     oj (3.10.8)
     padrino-helpers (0.14.4)
       i18n (~> 0.6, >= 0.6.7)
@@ -166,6 +174,10 @@ GEM
     pry (0.13.1)
       coderay (~> 1.1)
       method_source (~> 1.0)
+    pry (0.13.1-java)
+      coderay (~> 1.1)
+      method_source (~> 1.0)
+      spoon (~> 0.0)
     pry-rescue (1.5.2)
       interception (>= 0.5)
       pry (>= 0.12.0)
@@ -226,7 +238,7 @@ GEM
     simplecov (0.19.0)
       docile (~> 1.1)
       simplecov-html (~> 0.11)
-    simplecov-html (0.12.2)
+    simplecov-html (0.12.3)
     sinatra (2.1.0)
       mustermann (~> 1.0)
       rack (~> 2.2)
@@ -235,14 +247,20 @@ GEM
     slim (4.1.0)
       temple (>= 0.7.6, < 0.9)
       tilt (>= 2.0.6, < 2.1)
+    spoon (0.0.6)
+      ffi
     stream (0.5.2)
     sys-uname (1.2.1)
       ffi (>= 1.0.0)
     temple (0.8.2)
+    therubyrhino (2.1.2)
+      therubyrhino_jar (>= 1.7.4, < 1.7.9)
+    therubyrhino_jar (1.7.8)
     thor (1.0.1)
     thread_safe (0.3.6)
+    thread_safe (0.3.6-java)
     tilt (2.0.10)
-    timers (4.3.0)
+    timers (4.3.2)
     tzinfo (1.2.7)
       thread_safe (~> 0.1)
     uglifier (4.2.0)
@@ -254,6 +272,7 @@ GEM
     zeitwerk (2.4.0)
 
 PLATFORMS
+  java
   ruby
 
 DEPENDENCIES
@@ -266,6 +285,7 @@ DEPENDENCIES
   haml (>= 4.0.5)
   kramdown (~> 2.3)
   liquid (>= 3.0)
+  listen!
   middleman-cli!
   middleman-core!
   mini_racer (~> 0.3.1)