Add Python 3.6 support.

[camporter]

Adds Python 3.6 support by including it in the tox environments list, and updating travis configuration to use the tox environment. The jsonraw functionality missed using the cls keyword argument for the class to encode JSON with, so that has also been fixed so that tests in 3.6 pass.

[coveralls]

Coverage remained the same at 82.212% when pulling e580a8a on camporter:python3.6_support into f65c306 on softlayer:master.