php: security bump to version 5.5.21

Fixes:
CVE-2015-0231 - Use After Free Vulnerability in PHP's unserialize()
CVE-2014-9427 - Out of bounds read crashes php-cgi
CVE-2015-0232 - Free called on unitialized pointer

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>