Run Travis-CI build on pushes to all branches, not just master.

We can add a blacklist (rather than the previous whitelist) as needed
when we get newbors running.
Merge pull request #3043 from servo/travis-all-the-branches!

Run Travis-CI build on pushes to all branches, not just master.
Have http://servo.github.io/servo/ include docs for the Rust version we use.

Closes #3038.