Upgrade XDebug, always enable, switch from apt to PECL

* Upgrade XDebug from 2.1.0 to 2.2.3.  Fixes at least one bug with
  bogus CLASSNAME entries showing instead of object members in some
  cases.
* Always enable XDebug; it is no longer a role.
* Use the PECL package.  It's suggested prominently at
  http://xdebug.org/docs/install and is more up to date than Ubuntu's
  package.

Change-Id: Id564d627f8a4f26731223f3fd9181a3fabd5a54f