User.last_login should be set on _create_user

After django 1.7.0 `User.last_login` has changed from [`NOT_NULL` to `NULL`](https://code.djangoproject.com/ticket/7220).
When working on an existing project which has an initial migration setting `last_login` to `default=timezone.now` we'll have an `IntegrityError` every time we will try to create an user as the django migration for `last_login` would be overwritten by our initial migration.
Pass the function, not the time.