Added .travis.yml file

I added .travis.yml to the repository, which allows Travis-CI to see if it builds. Since it uses cmake to generate the build files, it shouldn't add any extra maintenance and you'll be able to confirm that pull requests build successfully before pulling them.

Here's it building my fork: https://travis-ci.org/vheuken/SFML

If you don't want this, you're free to close this pull request.