Test case demonstrating relation failure.

https://github.com/carljm/django-model-utils/issues/120
Ensure we call get_query_set on django 1.5-

Supersedes https://github.com/carljm/django-model-utils/pull/106

I can't actually see why this works, but it does: tests pass
after this that failed before.
Merge pull request #121 from schinckel/master

Make related query sets work on older django.