Replace Heapster with metrics-server once it is mature enough to use

[mjm]

As @Lyoness brought up in https://github.com/travis-ci/kubernetes-config/pull/16#issuecomment-439123009, Heapster is deprecated. We're using it for now because it integrates with other tools better, but presumably metrics-server will eventually do that. When that time comes, we should migrate.

[mjm]

From some experimentation with Kubernetes from personal projects, I've learned that metrics-server does work for powering the kubectl top command. My understanding is still that it has a new model for getting metrics out of Kubernetes, so I think to continue using statsd for Librato, we should stick with Heapster for now.