fix(builder): Non-root slugrunner and slugbuilder

Today apps are build as root and are running as root on deis, but heroku runs them as a non-root user. This removes this difference and fixes missing environment variables in `deis run`.
I saw this change when looking into the upstream slugrunner https://github.com/flynn/flynn/commit/c3d1a7d182847196d228acceccf42b3c4eff4f73 and think it is generally a good idea http://stackoverflow.com/a/18351547