Use the new build env on Travis

more capacity, faster vms, better boot times

http://docs.travis-ci.com/user/workers/container-based-infrastructure/

Closes #1586.
Cache pip downloads on Travis

The newly released container infrastructure apparently allows caching
for public repos (it used to be private only). So let's save some
bandwith and speed things up by populating the pip download cache.

Nicked from:
http://stackoverflow.com/questions/19422229/how-to-cache-requirements-for-a-django-project-on-travis-ci