Touch cookbook instead of saving

:fork_and_knife: We used to set some cookbook attributes in `Cookbook#publish_version` but no longer do. We left `save!` in to bump `Cookbook#updated_at` when a new version was published but it turns out this doesn't happen if no attributes are changed so replace it with touch.
Just confirming: is the semantically-most-recent the one that we want? Or do we want the chronologically most recent?