Update xINSTALL.centos7.txt
Update xINSTALL.centos7.txt

added  `rh-php56-php-opcache` as part of the `yum install` for CentOS7
FloatingGhost/MISP,devnull-/MISP,TheDr1ver/MISP,devnull-/MISP,MISP/MISP,SteveClement/MISP,Rafiot/MISP,Rafiot/MISP,TheDr1ver/MISP,FIRSTdotorg/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP,FloatingGhost/MISP,SteveClement/MISP,FIRSTdotorg/MISP,MISP/MISP,devnull-/MISP,SteveClement/MISP,MISP/MISP,devnull-/MISP,MISP/MISP,FIRSTdotorg/MISP,Rafiot/MISP,FloatingGhost/MISP,TheDr1ver/MISP,TheDr1ver/MISP,FIRSTdotorg/MISP,FloatingGhost/MISP,Rafiot/MISP