fix(docker): always use ":latest" tag in docker pull

Now that the Docker Index allows tagged builds directly from GitHub
project tags, we will version Deis components properly as
**deis/controller:v0.9.0** rather than **deisreleases/controller-v0.9.0**.
At that point, doing a plain "docker pull deis/controller" will pull
down all tagged images when we only wanted one, so this change
prevents users from wasting more time on already slow download operations.
Being explicit about the docker image tag also facilitates our
release process.