[racket-dev] Using Travis CI for Racket

I've just committed support for building Racket automatically on the
Travis continuous integration service.  See
https://travis-ci.org/samth/racket/ for the current build state.  This
doesn't track the actual `plt/racket` repository yet [1], but once it
does, it will do the following:

 - Build the core of Racket on every push (comparable to the "Minimal
Racket" snapshot)
 - Run the tests relevant to that core.

That's it -- this is a free service, and you can't take too much of
their time.  This isn't a replacement for DrDr, which runs _very_
comprehensive tests. But it provides very fast turnaround, multiple
compilers, and eventually OS X and Windows builds. Initially, no one
will get email -- let me know if you want to be emailed about build
results.

Currently, it runs tests for the following:

 - everything in `tests/racket/`
 - `json`
 - `xml`

Unfortunately, that `pkg` tests seem to break right now.

Let me know if there's something else that should be run to test that
"Minimal Racket" is working.

Sam

[1] That should happen once Eli is back to flip the relevant
configuration switch.