Add Travis Continuous Integration service to Markus

Travis CI is a free service used by a lot of open source projects that will automatically run builds in the cloud and report on success/failure of tests. For example, it will update Pull Requests to show whether it is passing or has broken tests.

To install (http://about.travis-ci.org/docs/user/getting-started/):
- Merge this PR
- Create a Travis CI account
- Activate the GitHub service hooks 

I think this would be a useful tool to get students thinking about tests, and to cover the possibility of tests passing locally for someone but not for others.