Fix bogus keyword argument passed to django_redis's DefaultClient.

Hi :-)
I ran into an issue using your lock implementation today. The keyword argument (`ex`) used to inform the timeout duration to django_redis's DefaultClient doesn't exist in the function definition. I checked earlier versions of django_redis to see whether the issue could have been caused by a version incompatibility, but it seems that the `ex` kwarg never existed. Please, could you verify?