Change embedded ids attribute name to match new version.

I think the new version of active_model_serializers appends '_ids' to an
embedded association. This makes all tests pass again.
Explicitly define as_json, to_json, serializable_hash.

Let's see if this fixes issue in Rails where to_json calls the super's
version of the method and returns an 'object' and 'options' key as
derived from the serializer object's attributes.
Merge pull request #2 from teejayvanslyke/master

Fixes for current active_model_serializers version with Rails