Fix tests for django 1.7

According to the docs https://docs.djangoproject.com/en/dev/releases/1.7/

> If you’re using Django in a plain Python script <...> you must now explicitly initialize Django at the beginning of your script with:
>
> ```
> >>> import django
> >>> django.setup()
> ```

> Otherwise, you will hit an `AppRegistryNotReady` exception.

Travis CI builds for DJANGO=dev fail right with this exception.