Treebeard update

Treebeard 2.0 was recently released with Matthews patches included. This pull request updates wagtail to use Treebeard 2.0 from pypi and removes our custom patched version.

The main difference is the add_child method has changed. Instead of passing the instance as the first argument you now have to pass the instance through with a keyword argument called "instance". Commit 9ba91c7 updates wagtail to use the new method.