Squashed 'wagtail/vendor/django-treebeard/' content from commit 15728a8

git-subtree-dir: wagtail/vendor/django-treebeard
git-subtree-split: 15728a88e71f3493e5a2829eb98563bd3bf03f17
Merge commit '4cce28b1f58bee3a2e698ef110fd855571704667' as 'wagtail/vendor/django-treebeard'
Merge branch 'master' of github.com:torchbox/wagtail
temporarily fiddle pythonpath to import our bundled django-treebeard