Fix regression introduced by PR #36 (ConfigurationInterface)

Sorry to say, but I've introduced a bug with my previous PR #36 . It just showed up on a composer install of version 1.0.3.

I've replaced ``getConfigTree`` with ``getConfigTreeBuilder``, but since my local test went ok (damned APC) I though it was just a deprecated method. It was not, it was called in ``OrnicarApcExtension``. I've aligned also that class.

Sorry for the double release.