Add testing for Python 3.6 and document support

[jdufresne]

No description provided.

[jdufresne]

Updated PR to resolve test failures after a recent rebase by removing

pip install pip==6.1.0

from requirements-install.sh

[gsnedders]

What's the default pip on Travis nowadays? It used to be 6.0.something, which didn't work for us. (See #319.)

[jdufresne]

AFAICT, It is not very well documented. This is what I found through experimentation:

Python 2.6.9
pip 9.0.1
setuptools 28.8.0

Python 2.7.13
pip 9.0.1
setuptools 36.3.0

Python 3.3.6
pip 8.1.2
setuptools 18.0.1

Python 3.4.6
pip 9.0.1
setuptools 35.0.2

Python 3.5.3
pip 9.0.1
setuptools 36.3.0

Python 3.6.3
pip 9.0.1
setuptools 36.5.0

Python 2.7.13 (c925e7381036, Jun 05 2017, 21:20:51)
[PyPy 5.8.0 with GCC 6.2.0 20160901]
pip 9.0.1
setuptools 36.2.3

Python 3.5.3 (a39af0be3a22, Jun 05 2017, 20:18:00)
[PyPy 5.8.0-beta0 with GCC 6.2.0 20160901]
pip 9.0.1
setuptools 36.2.3

[gsnedders]

Thanks!