Use nokogiri 1.6.x on Ruby 2.0

The nokogiri 1.7 requires Ruby version >= 2.1.0. Therefore, lock the version.

Author: y-yagi

gemfiles/Gemfile-rails-4-2

@@ -6,3 +6,6 @@ gemspec path: ".."
 gem "rails", "~> 4.2.0"
 gem "sprockets", "~> 4.0.0.beta2"
 gem "sprockets-rails", "~> 3.0.4"
+if RUBY_VERSION < '2.1.0'
+  gem 'nokogiri', '~> 1.6.8'
+end