docs(releases): update for Docker Index tagging capability

We no longer need to use the "deisreleases/component-vX.Y.Z" hack
to version Deis images, instead it's "deis/component:vX.Y.Z". Many
thanks to the Docker team for implementing this essential feature!
Merge pull request #1055 from deis/update-releases-doc

docs(releases): update for Docker Index tagging capability