Use install hook for supervisord installation

Current implementation tries to `apt-get install supervisor` on each push, which is obviously redundant. Supervisord should be installed using the `install` hook (via `dokku plugins-install`) and then run through docker.

This change should use a proper `install` hook using a minimal repo with a Dockerfile for supervisord (you can see the repo at https://github.com/yuvadm/dokku-supervisord-dockerfiles).

It *should* be working, but I'm still stuck on a minor bug that makes things not work, probably due to my inexperience with docker.

Any tips on how to complete this task?

(Testing this PR can be easily done by amending the `git clone` command to run from my fork - `yuvadm`).