[WIP] Enable travis builds for edx-platform

Currently enabled and building on my fork.
See: https://travis-ci.org/jzoldak/edx-platform/builds

@clytwynec @benpatterson FYI

Still needs:
- Documentation for how a repo owner would set up their own S3 buckets for the pip-accel cache
- Get the lms python tests passing. Travis is having trouble with how bulk email pipes to lynx to convert from html to plaintext. We should refactor that code anyhow.
- Get the lettuce tests passing. This requires Chromedriver/Chrome to be set up. However Chrome seems like it doesn't work correctly on the travis infrastructure. So that leaves 2 other choices: either get the lettuce tests working on SauceLabs (though this would be problematic for forks), or get them running in FireFox. (Or a third choice would be to convert them to Bok-Choy but that adds a lot of scope).
- Get the bok-choy tests sharded out in the matrix so that each one will run < 50min and get them all running.