RemovedInDjango20Warning - update for deprecation/changes

[alexlittle]

1. RemovedInDjango20Warning: Old-style middleware using settings.MIDDLEWARE_CLASSES is deprecated. Update your middleware and use settings.MIDDLEWARE instead.
2. django-oppia/oppia/views.py:48: RemovedInDjango20Warning: Using user.is_authenticated() and user.is_anonymous() as a method is deprecated. Remove the parentheses to use it as an attribute.
3. models.py:56: RemovedInDjango20Warning: on_delete will be a required arg for ForeignKey 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/1.11/ref/models/fields/#django.db.models.ForeignKey.on_delete

[alexlittle]

Fixed in python3django2 branch (https://github.com/DigitalCampus/django-oppia/tree/python3django2) - will be attached to proper version no once we know in which release we'll switch to python 3 and django 2