Enabling travis, partly

The basic idea is that when you update the repo, you get a version on https://fluffy.github.io/w3c-screen-share/

You'd need to turn travis on and add the secure token.  I can't do the former as a non-owner.  See https://github.com/martinthomson/i-d-template for details.  If you can turn on builds, I can do the rest, including testing and tweaking.