fix(buildpack): fix permissions issue with domain socket

This fix is deis-specific.

Use root for php-fpm to work around a PHP-specific issue when using
nginx/apache to server PHP apps in deis. The permissions of the fastcgi
domain socket are incorrect (760 instead of 766 or 777 like they should
be) and php-fpm / apache / nginx both do the Correct Thing and drop
elevated permissions as soon as they can. However, dropping when they do
you get a permission error akin to:

    [proxy:error] [pid 79:tid 140349649442560] (13)Permission denied:
        AH02454: FCGI: attempt to connect to Unix domain socket
	/tmp/heroku.fcgi.5000.sock (heroku-fcgi) failed

This commit addresses this issue and fixes the PHP buildpack to work
inside deis.

Previously this fixed the buildpack by changing the user to root. After
researching, a more correct solution is to change the permissions on the
domain socket.