Add a .travis.yml file

This is a reduced build that should finish within the 50 minute time
limit most of the time. In particular,
 * it skips DPH
 * it does not build dynamic libraries and no dynamic executables
 * does not build haddock or generate documentation
 * only runs fast tests, and no performance tests

You can see its results at
https://travis-ci.org/ghc/ghc/builds