* Test against Python 3.4 and multiple Twisted versions
This commit adds tox to be able to test against different dependencies
locally. We agreed that Python 3.4 should be supported across all Channels
projects, so it is also added with this commit.
Furthermore, I think it makes sense to support a broad range of Twisted
releases, as users of daphne are not unlikely to have other Twisted code
running. It's not feasible to test against all releases since 16.0, and
it would require constant maintenance to add new releases as they come
out. So I opted to keep things simple for now, and only test against the
oldest supported and the current Twisted release.
I did consider @jpic's great idea from
https://github.com/django/daphne/pull/19 to just use tox to avoid having
to duplicate the dependency matrix. But it does lead to slower test runs
as it bypasses Travis' caching, and is slightly more verbose.
* Require asgiref 1.0 and use receive instead of receive_many
As both daphne and asgiref had a 1.0 release, I think it makes sense to
require the presumably more stable asgiref 1.0. It's also a good
occasion to fix the deprecation warnings when running the tests by
switching to receive instead of receive_many.
* Document supported Python and Twisted 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.
