Merge pull request #1625 from progit/dependabot/bundler/html-proofer-3.18.8

Update html-proofer requirement from 3.18.6 to 3.18.8

Author: ben

Gemfile

@@ -13,4 +13,4 @@ gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.1.0'
 gem 'thread_safe', '0.3.6'
 gem 'epubcheck-ruby', '4.2.4.0'
-gem 'html-proofer', '3.18.6'
+gem 'html-proofer', '3.18.8'