Add .travis.yml configuration

This adds a configuration file for Travis CI, as discussed on #20. To make sure the tests actually pass, first pull request #27 should be merged.

For an example output, see
  https://travis-ci.org/fingolfin/gitifyhg/builds/4452561

Note that travis normally ships git 1.7.9.5. I vaguely recall that Felipe made some fast-export fixes at the time he submitted his remote-hg for git.git/contrib, which were meant to address issues that affect remote helpers. So it might be a good idea to require git 1.8.0 (which introduced them).

But in any case, the travis setup should likely also test against multiple git versions. For this, setting up a PPA with all desired git versions in it might be the best way to go.