(infra) Switch to container-based jobs on Travis-ci
(infra) Switch to container-based jobs on Travis-ci

Using cache for .m2 to avoid subsequent dependency downloads
See http://docs.travis-ci.com/user/caching/

lambdamatic/lambdamatic-project,lambdamatic/lambdamatic-project,xcoulon/lambdamatic-project,xcoulon/lambdamatic-project