Inspired by a similar change in the Kubernetes project [1], this changes the copyright header to the more generic "The rkt Authors" rather than "CoreOS, Inc.", which is only sometimes correct. https://github.com/GoogleCloudPlatform/kubernetes/commit/6b3a6e6b983f967c88d14d26542ec6e30c49ebd3
The cas package implements more than just a CAS, so this patch gives it a suitably generic name. Fixes #707