only enable waitress IPv6 support if we should serve on AF_INET6

Add option to disable monkeypatching.

Closes: #82

Convert readthedocs link for their .org -> .io migration for hosted projects

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.

[eventlet] Enable for Python 3

Use new Travis CI build env

instead of custom travis hooks

the geventwebsocket module (used by several backends) do use
collections.OrderedDict which is not available in python 2.6.
Thanksfully, there is a bw compat module available on pypi (ordereddict).

in chaussette/backend/_tornado.py and examples

in the source distribution (python setup.py sdist)

and fix the quick start section of the README file accordingly.

seems dead, see https://github.com/badges/pypipins/issues/37

Used to fail since tornado.wsgi.WSGIApplication inherits from
tornado.web.Application.