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.
I suppose for this purpose in that it relates to when it was published, probably chronologically the most recent, I'll adjust.