Fix broken reprs on notification models

Was getting things like this poking around in the shell:

>>> u.notification_methods.all()
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/opt/openduty/venv/local/lib/python2.7/site-packages/django/db/models/query.py", line 74, in __repr__
    return repr(data)
  File "/opt/openduty/venv/local/lib/python2.7/site-packages/django/db/models/base.py", line 423, in __repr__
    u = six.text_type(self)
TypeError: coercing to Unicode: need string or buffer, int found