Also run Travis CI tests on Python 3.6

I keep running into issues with tests passing locally (on Py36)
but failing remotely (on Py35).

To try to get some clarity on whether these things are definitely
a 3.5 vs. 3.6 thing, let's also run the CI tests on Python 3.6.
(a good idea anwyays as we probably want to upgrade at some point).