Basic Travis CI setup

I made a basic travis.yml file for #652. The build finishes well within their stated time limit of 50 minutes (or 10 minutes with no log output). I tested it, and it fails when there are syntax errors in the JavaScript, or documentation generation. I wasn't sure how to get documentation generation to generate warnings, so I don't know what will happen with that.

For it to work you have to follow the instructions at: http://about.travis-ci.org/docs/user/getting-started/

You can see the output from travis-ci for this branch here: https://travis-ci.org/cmorse/cesium/builds/8624560