XCache is no longer a zend extension.

Fixes issue where xcache would not load. It would fail with the following message:
`/usr/local/Cellar/php55-xcache/3.1.0/xcache.so doesn't appear to be a valid Zend extension`

More info here: https://groups.google.com/d/msg/xcache/UZFaC4iQSmc/5q6aLEKAWHsJ