Remove Rails, Sprockets and ActionMailer dependencies
Disable features broken in JRuby Nokogiri

See Nokogiri bugs #984 and #985:
  https://github.com/sparklemotion/nokogiri/issues/984
  https://github.com/sparklemotion/nokogiri/issues/985

This makes the JRuby build green again.
MAke JRuby officially supported
Remove code that shouldn't be in the spec_helper
Add experimental Rubinius support
Update changelog.
Integration specs showing that URLs from external assets are rewritten
Improve guard-rspec configuration