Switch to new Travis CI build environment

- faster VM boot time (running in a docker container)
- better performances (linux kernel 3.x instead of 2.x, 4G of RAM
  instead of 3G, better CPU power/parallelism, faster download, etc.)
- Ability to cache files in S3 storage

After file cache being filled, the total build time is reduced from ~6-7 minutes per job to [~5 minutes per job](https://travis-ci.org/gildegoma/scalaz/builds/45020812).

See also http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/.