Cleanup association to not use activerecord-deprecated_finders

Now that rails 4.2 :tada: is in beta we can start removing `activerecord-deprecated_finders` from the code. This cleans up the association changing `valid_options` to a constant (`VALID_OPTIONS`), removing the extra model argument from initialize and bumping `scope.is_a?(Hash)` where it belongs in `create_builder`.

Note this is for Rails 5 not Rails 4. :smile_cat: 

cc/ @tenderlove