fix(runner): add daemon user to root group

This is a hacky workaround to allow heroku-buildpack-php to work again. This should be removed once flynn/slugrunner#13 is addressed.

TESTING: `nsenter` into deis-builder and re-build `deis/slugrunner` with this branch:

    $ git clone -b hack-daemon-root https://github.com/deis/slugrunner
    $ cd slugrunner
    $ docker build -t deis/slugrunner .

After re-building, push example-php to deis:

    $ cd example-php
    $ deis create
    $ git push deis master

You should see a CodeIgniter welcome page.