[Feature] Clean up all but the current release

This PR adds the `--clean-all` option to both the `deploy` and `deploy:cleanup` commands.

When used, Rocketeer will prune all but the current release from the remote server (basically overriding the value of `remote.keep_releases`).

(Personally, I was using this when testing the ACL stuff.  It was a pain to have to deploy my app 4 times just to see if the pruning worked with the new perms.  This way, I can immediately prune old releases.  I figure the feature might be useful to others for other reasons as well.)