Prevent get_models() from returning abstract models on Django 1.7+.
Prevent get_models() from returning abstract models on Django 1.7+.

The compatibility implementation for `get_models()` wasn't filtering out
abstract models, leading to some problems with evolving apps that used
them. This updates the implementation to filter these out.

Testing Done:
Unit tests pass on Django 1.6 through 1.11.

The problem I hit in real-world usage was fixed.

Reviewed at https://reviews.reviewboard.org/r/10258/

beanbaginc/django-evolution