travis: Initial import of travis instructions.

travis: Initial import of travis instructions.

This just builds/installs our dependencies, and runs "make check".  I'm
interested in integrating more tests into it, but this seems like a pretty
easy first start.

If your personal branches of Mesa are on github, you can enable it on your
account and the repository (see
https://docs.travis-ci.com/user/for-beginners), then any pushes you do
will get their HEAD commit tested, and any pull requests to your tree will
get their merge commits tested.