Added travis-ci info to build project in a Docker container.

Related articles:
- http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
- http://docs.travis-ci.com/user/workers/container-based-infrastructure/
Merge pull request #249 from DWiechert/247-travis-ci-docker

Added travis-ci info to build project in a Docker container.