Add HHVM support for CentOS
HHVM is currently supported on Debian and Ubuntu only. The reason is that our HHVM implementation uses socket based connections and the apache 2.4 from CentOS is too old and does not support socket connections for mod_proxy_fcgi yet (requires apache 2.4.9, CentOS 7.2 uses 2.4.6) and the alternative to use mod_fastcgi in apache is not available for CentOS as there is no module. So either we implement port based connects for HHVM or we wait until CentOS updates it's apache.