change default redis db from 1 to 0

This will prevent redis-py from sending SELECT calls on each connection
and make the library compatible, by default, with twemproxy.
Compatible with Django 1.7

`django.core.cache.parse_backend_uri` was dropped in [1],
so the code is now included.

[1]: https://github.com/django/django/commit/4e9f800742c3048402a