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
Merge pull request #867 from gildegoma/new-travis-env

Switch to new Travis CI build environment