Add Travis

Summary:
To receive emails when tests fail, and to see build status on PRs.

You (or someone else with admin access) will have to enable the build at
https://travis-ci.org/profile/Khan.

Test Plan: * it worked here: https://travis-ci.org/justinj/react-components/builds/23463623

Reviewers: joel

Reviewed By: joel

Differential Revision: http://phabricator.khanacademy.org/D8409