* Mark runtests helper executable
* Bump required version of asgiref
We're probably making our life easier when we keep the Channels
dependencies roughly in sync. As a 1.0 release was made, I suggest to
require it.
* Simplify tox and Travis configuration
I hopefully simplified the tox configuration by following what I did in
the other four Channels projects.
I then had a good look at tox-travis and decided to remove it. It does
add a layer of indirection with, IMHO, not enough gain. To understand
what Travis is doing, one would need to consider two files (and
understand tox-travis) instead of just one file. It also introduces
another point of failure. What pushed me over was that there's a bug
with env matching (https://github.com/ryanhiebert/tox-travis/issues/55)
and tox or tox-travis seem to mask an Exception
(https://travis-ci.org/django/channels/jobs/195950971#L195) that would
be hard to debug.
The draw back is that we duplicate the Django dependency matrix, and the
commands that are executed in Travis and tox. We could add a "--with-qa"
flag to runtests.py to have it execute flake8 and isort to rectify the
latter.
I extracted test dependencies as I did for asgi_redis.
* Document supported versions
As per [their blog post of the 27th April](https://blog.readthedocs.com/securing-subdomains/) ‘Securing subdomains’:
> Starting today, Read the Docs will start hosting projects from subdomains on the domain readthedocs.io, instead of on readthedocs.org. This change addresses some security concerns around site cookies while hosting user generated data on the same domain as our dashboard.
Test Plan: Manually visited all the links I’ve modified.
