Update Ruby dependencies (#189)

This fixes a security vulnerability reported by GitHub's
automatic detection tool.

Gemfile.lock

@@ -1,87 +1,81 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    activesupport (5.2.2)
+    addressable (2.7.0)
-      concurrent-ruby (~> 1.0, >= 1.0.2)
+      public_suffix (>= 2.0.2, < 5.0)
-      i18n (>= 0.7, < 2)
-      minitest (~> 5.1)
-      tzinfo (~> 1.1)
-    addressable (2.6.0)
-      public_suffix (>= 2.0.2, < 4.0)
     colorator (1.1.0)
-    colorize (0.8.1)
+    concurrent-ruby (1.1.5)
-    concurrent-ruby (1.1.4)
     em-websocket (0.5.1)
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0.6.0)
     ethon (0.12.0)
       ffi (>= 1.3.0)
     eventmachine (1.2.7)
-    ffi (1.10.0)
+    ffi (1.11.1)
     forwardable-extended (2.6.0)
-    html-proofer (3.10.2)
+    html-proofer (3.12.2)
-      activesupport (>= 4.2, < 6.0)
       addressable (~> 2.3)
-      colorize (~> 0.8)
+      mercenary (~> 0.3)
-      mercenary (~> 0.3.2)
+      nokogiri (~> 1.10)
-      nokogiri (~> 1.9)
       parallel (~> 1.3)
+      rainbow (~> 3.0)
       typhoeus (~> 1.3)
       yell (~> 2.0)
     http_parser.rb (0.6.0)
-    i18n (0.9.5)
+    i18n (1.6.0)
       concurrent-ruby (~> 1.0)
-    jekyll (3.8.5)
+    jekyll (4.0.0)
       addressable (~> 2.4)
       colorator (~> 1.0)
       em-websocket (~> 0.5)
-      i18n (~> 0.7)
+      i18n (>= 0.9.5, < 2)
-      jekyll-sass-converter (~> 1.0)
+      jekyll-sass-converter (~> 2.0)
       jekyll-watch (~> 2.0)
-      kramdown (~> 1.14)
+      kramdown (~> 2.1)
+      kramdown-parser-gfm (~> 1.0)
       liquid (~> 4.0)
       mercenary (~> 0.3.3)
       pathutil (~> 0.9)
-      rouge (>= 1.7, < 4)
+      rouge (~> 3.0)
       safe_yaml (~> 1.0)
-    jekyll-redirect-from (0.14.0)
+      terminal-table (~> 1.8)
-      jekyll (~> 3.3)
+    jekyll-redirect-from (0.15.0)
-    jekyll-sass-converter (1.5.2)
+      jekyll (>= 3.3, < 5.0)
-      sass (~> 3.4)
+    jekyll-sass-converter (2.0.0)
-    jekyll-watch (2.1.2)
+      sassc (> 2.0.1, < 3.0)
+    jekyll-watch (2.2.1)
       listen (~> 3.0)
-    kramdown (1.17.0)
+    kramdown (2.1.0)
-    liquid (4.0.1)
+    kramdown-parser-gfm (1.1.0)
+      kramdown (~> 2.0)
+    liquid (4.0.3)
     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)
     mini_portile2 (2.4.0)
-    minitest (5.11.3)
+    nokogiri (1.10.4)
-    nokogiri (1.10.1)
       mini_portile2 (~> 2.4.0)
-    parallel (1.14.0)
+    parallel (1.17.0)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
-    public_suffix (3.0.3)
+    public_suffix (4.0.1)
+    rainbow (3.0.0)
     rb-fsevent (0.10.3)
     rb-inotify (0.10.0)
       ffi (~> 1.0)
-    rouge (3.3.0)
+    rouge (3.10.0)
     ruby_dep (1.5.0)
     safe_yaml (1.0.5)
-    sass (3.7.3)
+    sassc (2.2.0)
-      sass-listen (~> 4.0.0)
+      ffi (~> 1.9)
-    sass-listen (4.0.0)
+    terminal-table (1.8.0)
-      rb-fsevent (~> 0.9, >= 0.9.4)
+      unicode-display_width (~> 1.1, >= 1.1.1)
-      rb-inotify (~> 0.9, >= 0.9.7)
-    thread_safe (0.3.6)
     typhoeus (1.3.1)
       ethon (>= 0.9.0)
-    tzinfo (1.2.5)
+    unicode-display_width (1.6.0)
-      thread_safe (~> 0.1)
+    yell (2.2.0)
-    yell (2.0.7)
 
 PLATFORMS
   ruby
@@ -93,4 +87,4 @@ DEPENDENCIES
   tzinfo-data
 
 BUNDLED WITH
-   2.0.1
+   2.0.2