Merge pull request #1948 from progit/dependabot/bundler/html-proofer-5.0.9

Update html-proofer requirement from 5.0.8 to 5.0.9

Author: ben

Gemfile

@@ -14,5 +14,5 @@ gem 'coderay', '1.1.3'
 gem 'pygments.rb', '2.4.1'
 gem 'thread_safe', '0.3.6'
 gem 'epubcheck-ruby', '5.1.0.0'
-gem 'html-proofer', '5.0.8'
+gem 'html-proofer', '5.0.9'
 gem 'kindlegen', '3.1.1'