php: security bump to version 5.6.8

Fixes:
CVE-2015-1351 - OPCache: Use After Free
CVE-2015-1352 - Postgres: Null pointer dereference

And others with no CVE assigned yet.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>