Bump rack from 1.6.9 to 2.0.4

Bumps [rack](https://github.com/rack/rack) from 1.6.9 to 2.0.4.
- [Release notes](https://github.com/rack/rack/releases)
- [Changelog](https://github.com/rack/rack/blob/master/HISTORY.md)
- [Commits](rack/rack@1.6.9...2.0.4)

Signed-off-by: dependabot[bot] <support@dependabot.com>

Gemfile

@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-gem 'rack', '~> 1.6' # ~> 2.0 requires ruby 2.2.2 or later.
+gem 'rack', '~> 2.0' # ~> 2.0 requires ruby 2.2.2 or later.
 gem 'hikidoc'
 gem 'fastimage'
 gem 'emot'

Gemfile.lock

@@ -66,7 +66,7 @@ GEM
       byebug (~> 10.0)
       pry (~> 0.10)
     public_suffix (3.0.2)
-    rack (1.6.9)
+    rack (2.0.4)
     rack-test (0.8.3)
       rack (>= 1.0, < 3)
     racksh (1.0.0)
@@ -133,7 +133,7 @@ DEPENDENCIES
   octokit
   pit
   pry-byebug
-  rack (~> 1.6)
+  rack (~> 2.0)
   racksh
   rake
   redcarpet