Switch to new Travis CI build environment

The main advantages in favor of this switch are:
- comfortable file caching into S3 storage
- better perfs (4G of RAM instead of 3G, more modern VM containerization, linux kernel 3.x, more stable network, etc.)

@paulp for now, I let the git commits history to ease the review/selection. Please let me know if you whish something else (or differently written). If you accept all or some of these changes, I can rebase them into a single commit, of course.