Fix squlalchemy abstraction leakage

Sometimes a model is not a model and you can't set a model to None to
wipe it out, you have to set the relationship column id to None.

Also pin factoryboy so the tests run again,
Merge pull request #30 from jkoelker/when_a_sqlalchemy_model_is_not_a_sqlalchemy_model

Fix squlalchemy abstraction leakage