Passing None for the middleware get_response argument is deprecated.

[smithdc1]

Looking at the travis builds on master in addition to the url() deprecation there is also a few failing with this warning.

  /home/travis/build/encode/django-rest-framework/rest_framework/authentication.py:139: RemovedInDjango40Warning: Passing None for the middleware get_response argument is deprecated.
    check = CSRFCheck()

I couldn't see anything raised already but my search skills for url() let me down somewhat 🙈