Update DB selection syntax for Redis cache urls

[alorence]

Since version 0.4.1, it is now possible to correctly configure a Redis server as cache system for Django application using django-environ. Specifically, it is now possible to select the DB to use.

As described in django-redis documentation, the correct value for Redis LOCATION Url is redis://127.0.0.1:6379/1 for DB1, redis://127.0.0.1:6379/6 for DB6, etc.

In my tests, the syntax redis://127.0.0.1:6379:1 didn't work. It writes values to DB 0 by default, and in some case, raises an exception:

  [...]
  File "C:\Users\Antoine\.virtualenvs\xxx\lib\site-packages\django_redis\pool.py", line 114, in get_connection_pool
    pool = self.pool_cls.from_url(**cp_params)
  File "C:\Users\Antoine\.virtualenvs\xxx\lib\site-packages\redis\connection.py", line 813, in from_url
    'port': int(url.port or 6379),
  File "c:\python35-x64\Lib\urllib\parse.py", line 159, in port
    port = int(port, 10)
ValueError: invalid literal for int() with base 10: '6379:4'

This PR may fix the issue and help users to use the right syntax when declaring a Redis cache.

[coveralls]

Coverage remained the same at 88.312% when pulling a4b38a2 on alorence:develop into 43d572f on joke2k:develop.