Rails 4 compatibility

* also bumps the activerecord dependency requirement to be 3.0 or higher as delayed_job 3.x does not support activerecord 2.x
* because of the activerecord 3.0+ requirement change, the scoping done in backend/active_record.rb can be updated to use Rails 3 query methods
* also removes a spec that tested `attr_accessible` which is no longer part of Rails 4