Python 3.6 support

[impredicative]

I would like to request documented Python 3.6 support. This is of course defined in setup.py, and is reflected under Categories at https://pypi.python.org/pypi/jsonschema/

[Julian]

Hey, thanks!

Fine with me as long as we actually support 3.6 (at very least by making sure travis runs tests under it) -- care to send a PR?

[Julian]

Closed via #353