Update Stem's Travis CI config

[teor2345]

Once we fix Tor's Travis make test-stem timeouts, we should update Stem's Travis CI config, based on Chutney's Travis CI config.

Chutney's config provides a combination of OSes, Tor versions, Python versions, and PyPy versions. (We won't need the chutney-specific network tests.)

Here's the relevant Tor ticket: https://trac.torproject.org/projects/tor/ticket/29437

I'm happy to do this task, once we get Tor/Stem working in Travis CI again.

[atagar]

Thanks teor, this sounds neat. If you need anything from me let me know.