fix #1535 xdebug installation fails when php version is 5.6
fix #1535 xdebug installation fails when php version is 5.6

LaraDock/laradock,zuohuadong/laradock,jinnawat/laradock,bestlong/laradock,springjk/laradock,bestlong/laradock,Harmonic/laradock,schiggi/laradock,schiggi/laradock,hnquang112/laradock,bestlong/laradock,luciano-jr/laradock,bestlong/laradock,zuohuadong/laradock,cjnchk/laradock,AliveCorpse/laradock,arkadiusjonczek/laradock,arkadiusjonczek/laradock,jfoliveira/laradock,Harmonic/laradock,arkadiusjonczek/laradock,hnquang112/laradock,ralphowino/laradock,cjnchk/laradock,schiggi/laradock,cjnchk/laradock,zuohuadong/laradock,ralphowino/laradock,AliveCorpse/laradock,springjk/laradock,jinnawat/laradock,ralphowino/laradock,Harmonic/laradock,luciano-jr/laradock,tharaxodia/laradock,arkadiusjonczek/laradock,jfoliveira/laradock,Harmonic/laradock,springjk/laradock,jinnawat/laradock,hnquang112/laradock,hnquang112/laradock,jinnawat/laradock,springjk/laradock,zuohuadong/laradock,ralphowino/laradock,cjnchk/laradock,bestlong/laradock,tharaxodia/laradock,schiggi/laradock