ActiveRecord - With no submodules (core), it behaves like Rails 3 Core Model

[jonmchan]

https://github.com/Sorcery/sorcery/blob/master/spec/active_record/user_spec.rb

https://github.com/Sorcery/sorcery/blob/master/spec/shared_examples/user_shared_examples.rb

Replace rails_3_core_model with rails_core_model.