Change assertIsNone to assertTrue and is None
Change assertIsNone to assertTrue and is None

`unittest.TestCase` in Python 2.6 has not method called `assertIsNone()`. As much as I'd like to drop support for Python 2.6 (Heroku uses 2.7), I'd really like to keep support for 2.6 if possible. It's silly to drop it over tests.

dirn/django-heroku-redisify,dirn/django-heroku-redisify