No set_php5_fpm_sock_group_and_user() for hhvm

This looks like php-only code and shouldn't be run for hhvm. When it is run, it causes commands to be run like

    chmod 660  && chown www-data  && chgrp www-data

(these commands are missing the file argument)

Introduced here: https://github.com/puphpet/puphpet/commit/770aefd6e0d3654ef0bb35faa5c078c53ba2501c