It is very easy to use travis-ci w/ projects using MongoDB in unit tests. It took only minutes to prepare my github project ttrelle/spring-batch-mongodb-support.
1) I had to log in to travis-ci, sync my github projects and activate travis for the above mentioned project.
2) Wrote a .travis.yml file:
The 2nd option will provide a mongod process on port 27017 (default) during the unit tests.
3) Pushed to github to trigger a travis build.
Et voilá: https://travis-ci.org/ttrelle/spring-batch-mongodb-support