Upgrade treebeard to version 2.0 for Django 1.5 compatibility.

The django-treebeard version we were using, 1.61 (via KGS), is not compatible with Django 1.5. Version 2.0 supports Django 1.5 and seems to be backwards compatible. See changelog: https://tabo.pe/projects/django-treebeard/docs/tip/changes.html.