Some Travis adjustments

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | ---
| Fixed tickets | ---
| Refs tickets  | http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/
| License       | MIT
| Doc PR        | ---