Add .travis.yml for Travis CI

Travis CI is a hosted CI system that automatically runs your tests whenever you push new commits to GitHub.

For one-time setup instructions to set up your GitHub repo to work with Travis CI, see http://about.travis-ci.org/docs/user/getting-started/ (takes only a minute or two). 

Here's a sample Travis CI build for my repo that is passing:

https://travis-ci.org/msabramo/rxjson/builds/4674450