Remove TRAVIS environment variable from .travis.yml

You can see in [the Travis CI output](https://travis-ci.org/inkling/Subliminal/jobs/22304992) that Travis is now exporting the `TRAVIS` environment variable. I also echoed the variable to check its value in [this build](https://travis-ci.org/MaxGabriel/TestTravis/builds/22316709).

If this PR is merged, I'll also update the Subliminal [CI wiki page](https://github.com/inkling/Subliminal/wiki/Continuous-Integration).