Add Python 3.6 to the travis build

[behrtam]

We should validate and finally add Python 3.6 to the travis test setup. https://github.com/exercism/xpython/blob/master/.travis.yml#L9

[surajnarwade]

I would like to take this task.

[behrtam]

Looks like the nightly build works with our existing code base: https://travis-ci.org/exercism/xpython/jobs/168941309