Speedup travis builds

[staabm]

Activate the faster travis infrastructure and activate caching for npm dependencies between builds.

Speedup does not necessarily mean per build time, but also shorter waiting times in build queues.

[jzaefferer]

Thanks! I just updated the commit message. And while we're here: You're welcome to push branches that are likely long-lived to this repo, otherwise you can stick with creating branches on your repo. Not that it matters much either way.

[staabm]

Thanks, I try to remember it :D