Use Ubuntu package for xdebug

Installing via pecl is broken with hhvm taking over /usr/bin/phpize via
alternatives. We switched to the pecl version in Id564d62 to get version
2.2.3 on Ubuntu 12.04 and Ubuntu 14.04 ships with 2.2.3.

Change-Id: I91d51f6deb0b12b795f705cf8895783a11fa8695