Fix ImproperlyConfigured Exception for ModelForm

Fix ImproperlyConfigured Exception for ModelForm

The issue was caused by this change in Django 1.8:
In older versions, omitting both fields and exclude resulted in a form with all the model’s fields. Doing this now raises an ImproperlyConfigured exception.

See https://docs.djangoproject.com/en/1.8/topics/forms/modelforms/#selecting-the-fields-to-use