Loosen Nokogiri version requirement

Closes #13218

Gemfile

@@ -105,7 +105,7 @@ gem 'rouge',         '~> 1.10.1'
 # See https://groups.google.com/forum/#!topic/ruby-security-ann/aSbgDiwb24s
 # and https://groups.google.com/forum/#!topic/ruby-security-ann/Dy7YiKb_pMM
-gem 'nokogiri', '1.6.7.2'
+gem 'nokogiri', '~> 1.6.7', '>= 1.6.7.2'
 # Diffs
 gem 'diffy', '~> 3.0.3'

Gemfile.lock

@@ -959,7 +959,7 @@ DEPENDENCIES
   mysql2 (~> 0.3.16)
   nested_form (~> 0.3.2)
   net-ssh (~> 3.0.1)
-  nokogiri (= 1.6.7.2)
+  nokogiri (~> 1.6.7, >= 1.6.7.2)
   nprogress-rails (~> 0.1.6.7)
   oauth2 (~> 1.0.0)
   octokit (~> 3.8.0)