php: security bump to version 5.5.18

Fixes:
CVE-2014-3669 - Integer overflow in unserialize() (32-bits only)
CVE-2014-3670 - Heap corruption in exif_thumbnail()
CVE-2014-3668 - Global buffer overflow in mkgmtime() function

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Acked-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>