* Add travis testing support for python 3.8
* Use Xenial for all
* Simplify matrix
* Consistent indents
* Add py38 to tox and update contributing guide for how to use pyenv
* Fix travis spec to allow 3.8-dev to fail and tox should pass 90% coverage for full suite
When I ran the test suite on macOSX, I had some issues regarding SSL configuration, I have documented the error I encounter and the solution to deal with it.
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.
