py-channels: updated to 2.1.5

2.1.5:
* Django middleware caching now works on Django 1.11 and Django 2.0.
  The previous release only ran on 2.1.

2.1.4:
* Django middleware is now cached rather than instantiated per request
  resulting in a sigificant speed improvement
* ChannelServerLiveTestCase now serves static files again
* Improved error message resulting from bad Origin headers
* runserver logging now goes through the Django logging framework
* Generic consumers can now have non-default channel layers
* Improved error when accessing scope['user'] before it's ready

(adam)