Conforming to Django 1.6+ special timeout values

Django 1.6+: Explicitly passing in timeout=None will set a non-expiring timeout. Passing in timeout=0 will set-and-expire-immediately the value.

I didn’t know tox. The tox output was so verbose that I can’t tell if my commit passed the tests or not…
I believe so. In the `set` method you have:
```
    elif timeout is 0:
        # Django 1.6+: Passing in timeout=0 will set-and-expire-immediately the value.
        uwsgi_timeout = -1
```