Add travis yml - #1

I once asked the travis-ci guys to pre-install numpy, scipy and matplotlib because I also ran into scipy build timeouts with a project.
https://groups.google.com/d/topic/travis-ci/uJgu35XKdmI/discussion

You don't have to wade through the whole discussion, here's the summary:
They basically agreed to to it but then never got around to it.
Probably if more people express interest, they will do it. :-)

You could still test your python / numpy test matrix without problems, even if numpy came pre-installed, I think.
Just for the scipy and matplotlib part (if you have any tests there) you'd have at least one version to test against without running into the build timeout.

Closing this, as it has now been activated on the main astropy repo