try adding json gem dependency

jabenninghoff · jabenninghoff · commit f73988db217e · 2017-01-21T17:13:21.000-06:00
diff --git a/Gemfile b/Gemfile
@@ -7,3 +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
diff --git a/Gemfile.lock b/Gemfile.lock
@@ -209,6 +209,7 @@ PLATFORMS
 DEPENDENCIES
   github-pages (= 116)
   html-proofer
+  json
 
 RUBY VERSION
    ruby 2.3.3p222
