Adds configuration file for Travis CI

Travis CI is a hosted continuous integration service that is integrated with GitHub. With the added configuration, Travis CI builds can be enabled, so that on every push to the git repository a build is triggered automatically. It is even possible to build against different JDK versions. The build status can be displayed in the README as a small image. For further information see the [Travis CI Getting Started Guide](http://docs.travis-ci.com/user/getting-started/).