Resolve Django 2.0 deprecation warnings

[catskul]

env/local/lib/python2.7/site-packages/django_twilio/migrations/0001_initial.py:34: RemovedInDjango20Warning: on_delete will be a required arg for OneToOneField in Django 2.0. Set it to models.CASCADE on models and in existing migrations if you want to maintain the current default behavior. See https://docs.djangoproject.com/en/1.9/ref/models/fields/#django.db.models.ForeignKey.on_delete
  ('user', models.OneToOneField(to=settings.AUTH_USER_MODEL)),

[aljp]

Django 2.0 is out now and this hasn't been fixed. I've made an issue referencing the error raised by Django 2.0 #103

[jheld]

master and 0.9.0 release address this. Thank you.

If you encounter any issues please re-open.