Runs specs against Rails 4.
Clean things up to pass on Rails 4

* Test against Rails 4.0 and master
* Remove ActiveResource require - this wasn't used and was only part of
  the generated dummy spec
* Remove active_resource/railtie require in spec/dummy
* Can't test ruby 1.9.2 against rails 4, as that's not supported
  anymore.
* Remove whitelist_attributes