How to cache maven local repo between Travis builds

caching is allowed only for private repos or public repos that use "Using container-based infrastructure".

Steps:
1. instruct travis to use "container-based infrastructure"
2. instruct travis what folder you need to cache

Official documentation:
http://docs.travis-ci.com/user/workers/container-based-infrastructure/#Routing-your-build-to-container-based-infrastructure
http://docs.travis-ci.com/user/caching/#Arbitrary-directories


Results: that caching speedup build on ~3-4 minutes for each jdk.

Example: https://github.com/checkstyle/checkstyle/blob/master/.travis.yml