travis: enable container-based builds

Jobs running on container-based infrastructure:
1. start up faster
2. allow the use of caches for public repositories
3. disallow the use of sudo, setuid and setgid executables

http://docs.travis-ci.com/user/workers/container-based-infrastructure/
Merge pull request #524 from 3flex/container-builds-on-travis

travis: enable container-based builds