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.
This can safely be `cookbook.save(validate: false)`, which will drastically reduce the runtime of the migration (on my machine it cuts it from ~30s to ~10s).