Django1.7: Rename MP_NodeManager.get_query_set to get_queryset

Django 1.7 issues below warning:
treebeard/mp_tree.py:102: RemovedInDjango18Warning:
`MP_NodeManager.get_query_set` method should be renamed `get_queryset`.

So the method get_query_set has been renamed to get_queryset to avoid
this warning.