Remove hard dependency on nokogiri for Rails 4.2

Nokogiri 1.7.x requires Ruby 2.1+ so let bundler handle the version.

Author: pixeltrix

gemfiles/rails_4_2.gemfile

@@ -9,6 +9,5 @@ gem "pry"
 gem "railties", "~> 4.2.0"
 gem "actionpack", "~> 4.2.0"
 gem "activemodel", "~> 4.2.0"
-gem "nokogiri", "~> 1.6.8"
 
 gemspec path: "../"