Rails 4 Support

I think this should clean up the test suite for Rails 4, and I'm pretty comfortable with that code.

It also resolves https://github.com/lomba/schema_associations/issues/7 .  I'm a hair nervous since I don't totally understand the changes between Rails 3 and Rails 4 that resulted in the issue and I don't fully understand the code in question, but the change passed against the existing test suite without any issues.

I think my nervousness is more about whether there may be issues if the include for the scope specifies a model that has a default scope that specifies includes for the included model, but this is more of a vague idea.