php: upgrade to 5.5.15

Based on the corresponding changes to the modphp recipe in
meta-webserver. From the commit message for the modphp upgrade:

* LIC_FILES_CHKSUM needed to change because the copyright year changed
* Enabled mcrypt since upstream recommend this for acceptable
  performance
* Disabled the opcache; this is a new feature in 5.5 and the configure
  check for it currently breaks when cross-compiling.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>