added missing html-proofer dependency

Gemfile

@@ -8,6 +8,7 @@ source "https://rubygems.org"
 # This will help ensure the proper Jekyll version is running.
 # Happy Jekylling!
 gem "jekyll", "~> 4.0.0"
+gem "html-proofer"
 # This is the default theme for new Jekyll sites. You may change this to anything you like.
 # gem "minima", "~> 2.5"
 # If you want to use GitHub Pages, remove the "gem "jekyll"" above and

Gemfile.lock

@@ -8,9 +8,19 @@ GEM
     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 (1.12.2)
     forwardable-extended (2.6.0)
+    html-proofer (3.15.1)
+      addressable (~> 2.3)
+      mercenary (~> 0.3)
+      nokogumbo (~> 2.0)
+      parallel (~> 1.3)
+      rainbow (~> 3.0)
+      typhoeus (~> 1.3)
+      yell (~> 2.0)
     http_parser.rb (0.6.0)
     i18n (1.8.2)
       concurrent-ruby (~> 1.0)
@@ -41,9 +51,16 @@ GEM
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
+    mini_portile2 (2.4.0)
+    nokogiri (1.10.8)
+      mini_portile2 (~> 2.4.0)
+    nokogumbo (2.0.2)
+      nokogiri (~> 1.8, >= 1.8.4)
+    parallel (1.19.1)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (4.0.3)
+    rainbow (3.0.0)
     rb-fsevent (0.10.3)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
@@ -54,17 +71,21 @@ GEM
     terminal-table (1.8.0)
       unicode-display_width (~> 1.1, >= 1.1.1)
     thread_safe (0.3.6)
+    typhoeus (1.3.1)
+      ethon (>= 0.9.0)
     tzinfo (1.2.6)
       thread_safe (~> 0.1)
     tzinfo-data (1.2019.3)
       tzinfo (>= 1.0.0)
     unicode-display_width (1.6.1)
     wdm (0.1.1)
+    yell (2.2.2)
 
 PLATFORMS
   ruby
 
 DEPENDENCIES
+  html-proofer
   jekyll (~> 4.0.0)
   tzinfo (~> 1.2)
   tzinfo-data