Run plugins-install hooks in series

If several plugins install new packages with `apt-get`, `dokku plugins-install` breaks because `pluginhook` runs scripts in parallel, and `apt-get`cannot run in parallel (`dpkg` locks `/var/lib/apt/lists/lock`).
This happens as soon as we add a plugin using `apt-get` (like dokku-mongodb-plugin) since `nginx-vhosts` already calls `apt-get`.
This PR fixes it by running hooks in series (progrium/pluginhook#1 would be a nicer fix).