Test on and make packages for Python 3.6

Python 3.6 was released yesterday. We should add testing and packaging, and fix any problems with it, to:

• Linux wheel builds, depends on manylinux images supporting 3.6
• Linux tests on Travis
• OSX tests & wheel builds on Travis
• Windows tests & wheel builds on Appveyor

I added 3.6 to the Travis tests for Linux. When Travis ran, the OSX Py3 build automatically got Python 3.6, so the wheel is built for that. They both pass the tests.

Still waiting on manylinux and Appveyor.

This should all be working now.