upgrade jekyll; closes #3548 (#3549)

* upgrade jekyll; closes #3548

see https://nvd.nist.gov/vuln/detail/CVE-2018-17567

* add .ruby-version for netlify

.ruby-version

@@ -0,0 +1 @@
+2.4.3

Gemfile

@@ -1,3 +1,3 @@
 source "https://rubygems.org"
 # Added at 2017-12-05 23:01:55 -0800 by boneskull:
-gem "jekyll", "~> 3.6"
+gem "jekyll", ">= 3.8.4"

Gemfile.lock

@@ -14,7 +14,7 @@ GEM
     http_parser.rb (0.6.0)
     i18n (0.9.5)
       concurrent-ruby (~> 1.0)
-    jekyll (3.8.3)
+    jekyll (3.8.4)
       addressable (~> 2.4)
       colorator (~> 1.0)
       em-websocket (~> 0.5)
@@ -29,25 +29,25 @@ GEM
       safe_yaml (~> 1.0)
     jekyll-sass-converter (1.5.2)
       sass (~> 3.4)
-    jekyll-watch (2.0.0)
+    jekyll-watch (2.1.2)
       listen (~> 3.0)
     kramdown (1.17.0)
-    liquid (4.0.0)
+    liquid (4.0.1)
     listen (3.1.5)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
       ruby_dep (~> 1.2)
     mercenary (0.3.6)
-    pathutil (0.16.1)
+    pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (3.0.3)
     rb-fsevent (0.10.3)
     rb-inotify (0.9.10)
       ffi (>= 0.5.0, < 2)
-    rouge (3.2.1)
+    rouge (3.3.0)
     ruby_dep (1.5.0)
     safe_yaml (1.0.4)
-    sass (3.5.7)
+    sass (3.6.0)
       sass-listen (~> 4.0.0)
     sass-listen (4.0.0)
       rb-fsevent (~> 0.9, >= 0.9.4)
@@ -57,7 +57,7 @@ PLATFORMS
   ruby
 
 DEPENDENCIES
-  jekyll (~> 3.6)
+  jekyll (>= 3.8.4)
 
 BUNDLED WITH
-   1.16.1
+   1.16.6