Fix the tests for running on django 1.6 and 1.7

  - We can install with Django 1.6b1
  - We can't use files settings files named 'test_*' any more, with
    Django's new test discovery.
  - ModelForm.Meta.fields is required
Django #20272 is fixed, we can use our own EXISTS query

This fixes the case where there were no fields to update, so Django
short-circuited the `_do_update` method. Now, Django still calls it, so
we can add our version filter to the query.