Fix parse_qsl is undefined

Using the most recent build, I get this error :

File "/src/django-cache-machine/caching/invalidation.py", line 213, in get_redis_backend
    server, params = parse_backend_uri(settings.REDIS_BACKEND)
  File "/src/django-cache-machine/caching/invalidation.py", line 200, in parse_backend_uri
    params = dict(parse_qsl(rest[qpos+1:]))
NameError: global name 'parse_qsl' is not defined

This commit fixes the issue.