Install XDebug via PECL rather than apt

The version of XDebug provided by the Ubuntu repository was 2.2.3,
which (per #317) appears to cause some issues when attempting to
load our default sites on some machines.

Using PECL directly allows us to install the latest stable version,
which is 2.2.5 at this time.
Merge pull request #343 from Varying-Vagrant-Vagrants/bug/xdebug

[PR] Install XDebug via PECL rather than apt