Make work with Heroku style PAAS

I've been trying to get this to work on Dokku etc for a couple days and have finally cast the correct incantation.
This is mainly provided as a suggestion, feel free to cherry pick if you like.

The ```cffi``` requirement was added so that the buildpack would build it correctly.
Using a Procfile means we can get rid of the exec in run.sh again.
```
git clone https://github.com/stuart-warren/docker-registry.git
git remote add server1 dokku@dokku.server1.com:registry
git push server1 master

dokku config:set DOCKER_REGISTRY_CONFIG=config_sample.yml
```