Added html-proofer gem.

This will also add rake, which TravisCI is choking without.

Author: patcon

Gemfile

@@ -9,3 +9,4 @@ source "https://rubygems.org"
 # This will help ensure the proper Jekyll version is running.
 # Happy Jekylling!
 gem "jekyll", "~> 3.8.5"
+gem "html-proofer"

Gemfile.lock

@@ -1,16 +1,32 @@
 GEM
   remote: https://rubygems.org/
   specs:
+    activesupport (5.2.3)
+      concurrent-ruby (~> 1.0, >= 1.0.2)
+      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)
     concurrent-ruby (1.1.5)
     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 (1.11.1)
     forwardable-extended (2.6.0)
+    html-proofer (3.11.1)
+      activesupport (>= 4.2, < 6.0)
+      addressable (~> 2.3)
+      mercenary (~> 0.3.2)
+      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)
       concurrent-ruby (~> 1.0)
@@ -38,9 +54,15 @@ GEM
       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 (1.10.3)
+      mini_portile2 (~> 2.4.0)
+    parallel (1.17.0)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (3.1.0)
+    rainbow (3.0.0)
     rb-fsevent (0.10.3)
     rb-inotify (0.10.0)
       ffi (~> 1.0)
@@ -52,12 +74,19 @@ GEM
     sass-listen (4.0.0)
       rb-fsevent (~> 0.9, >= 0.9.4)
       rb-inotify (~> 0.9, >= 0.9.7)
+    thread_safe (0.3.6)
+    typhoeus (1.3.1)
+      ethon (>= 0.9.0)
+    tzinfo (1.2.5)
+      thread_safe (~> 0.1)
+    yell (2.2.0)
 
 PLATFORMS
   ruby
 
 DEPENDENCIES
+  html-proofer
   jekyll (~> 3.8.5)
 
 BUNDLED WITH
-   1.16.1
+   1.17.3