enable Travis Continuous Integration

This PR will prepare mesos-go to be built by [Travis CI](https://travis-ci.org).
After [enabling the project on Travis webpage](http://docs.travis-ci.com/user/getting-started/#Step-one%3A-Sign-in) by "mesosphere" group member, this will allow automatic building and testing of all pushed commits, as well as pending pull requests.
A [build status badge](http://docs.travis-ci.com/user/status-images/) has also been added to the repository's front page via README.md file.