Fix errors and warnings for Django 1.8
Fix errors and warnings for Django 1.8

These were the errors Django has reported:

"""
ERRORS:
django_cas_ng.ProxyGrantingTicket.date: (fields.E160) The options auto_now, auto_now_add, and default are mutually exclusive. Only one of these options may be present.

WARNINGS:
django_cas_ng.SessionTicket.session: (fields.W342) Setting unique=True on a ForeignKey has the same effect as using a OneToOneField.
        HINT: ForeignKey(unique=True) is usually better served by a OneToOneField.
"""

More information here:
https://code.djangoproject.com/ticket/21798#comment:11
https://docs.djangoproject.com/en/1.8/ref/models/fields/#datefield

pbaehr/django-cas-ng,mingchen/django-cas-ng,bgroff/django-cas-ng,nitmir/django-cas-ng,wrygiel/django-cas-ng,IRI-Research/django-cas-ng