Document travis fix

Author: jabenninghoff

Gemfile

@@ -7,4 +7,4 @@ version = JSON.parse(open('https://pages.github.com/versions.json').read)
 ruby version['ruby']
 gem 'github-pages', version['github-pages'], group: :jekyll_plugins
 gem 'html-proofer', group: :htmlproofer
-gem 'json', group: :travis_ci
+gem 'json', group: :travis_ci # requiring json a second time fixes Travis CI builds