Add Travis CI

This adds Travis CI to the repo.

Note: this only _fake_ adds it.  I added it to my branch to confirm that the `.travis.yml` file works, and that we're passing on all versions added:

![](https://www.evernote.com/shard/s233/sh/53c111f3-dcda-40a6-97d3-8c5686ea3a68/86c2ed961d0608f4cbe6190b677856da/res/96f6f0a8-3d53-4a83-a866-0c2123078b5e/Travis_CI_-_Free_Hosted_Continuous_Integration_Platform_for_the_Open_Source_Community-20130506-222914.jpg.jpg?resizeSmall&width=832)

You'll need to activate the GitHub service hook for the main repo, but that should be all that's necessary -- merging the pull req should fire the hook and trigger the first build.

See [the Travis getting started guide](http://about.travis-ci.org/docs/user/getting-started/) for more info, and here's [the page for my fork's builds](https://travis-ci.org/tjschuck/bcrypt-ruby) if you're curious.