Rails 4 compatibility

This PR should allow delayed_job_active_record to run on both Rails 3.x and 4.x, fixing #30. A few of the specific changes are listed below.

* 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
* Removes a spec that tested `attr_accessible` which is no longer part of Rails 4

Additionally, this will require collectiveidea/delayed_job#490 to be merged in. I would also figure that after this is merged, the dependency on `delayed_job` in `delayed_job_active_record.gemspec` will need to be bumped to something like `~> 3.1`, assuming the newest `delayed_job` release is tagged at 3.1.

Any comments/suggestions would be great!