Missing environment variables

[danilobellini]

Travis includes several useful environment variables to their builds:

https://docs.travis-ci.com/user/environment-variables/

The TRAVIS_PYTHON_VERSION is what I missed the most, but every language has a language-specific environment variable. These are useful when installing some version-specific stuff or for calling a specific test script (e.g. when installing miniconda).

[guyzmo]

I actually have that patched in my local tree. I still need to make the commits and push it, though ☺