Use new Docker infrastructure on Travis CI

[franzliedke]

Once my pull request to Travis is deployed (supposedly early April 2015), we can switch back to using the Docker-based infrastructure, as that will then already have Python 3 installed for us. =)

[franzliedke]

Can be merged in two days, when this is deployed to Travis:
travis-ci/docs-travis-ci-com#224

[franzliedke]

Postponed to next week: http://docs.travis-ci.com/user/build-environment-updates/2015-04-09/