Incompatible with channels 3

[OmarWKH]

Related to #56.

Websocket breaks when channels 3 is installed. Channels 2.4.0 works.

[EJH2]

Websocket breaking in channels v3 seems to stem from this issue. This can be fixed by adding the websocket url in routing.py as such:

urlpatterns = [
    url(r'^ws/progress/(?P<task_id>[\w-]+)/?$', consumers.ProgressConsumer.as_asgi()),
]

Now, either we can do that and drop support for channels v2, or do a try/except clause and support both. Thoughts @czue @OmarWKH?

[czue]

I guess if there's no major downside and it's a simple try/catch then supporting both for now seems better?