Remove socket when overmind process is stopped.

[henrikbjorn]

This should fix #186 by cleaning up the socket when the overmind process is stopped.

It dosen't seem to be any tests in the repo so I tested it manually with the following Procfile

web: while true; do echo 'Hello, World!'; sleep 1; done

Then verifying the socket is present, and after ctrl+c that it is gone.