[enh] Migrating Travis CI from legacy to container-based infrastructure

This patch should speedup testing with Travis CI using caching and
container-based infrastructure.

source:
- http://docs.travis-ci.com/user/migrating-from-legacy/
- http://docs.travis-ci.com/user/caching/

.travis.yml

+sudo: false
+cache:
+  - pip
+  - npm
 language: python
 python:
   - "2.7"