Dependency update: Support django-treebeard 3.0 and 4.0 releases

The admin extensions in django treebeard 3.0 are not compatible with
Django 1.9. Although these are not used by Wagtail they are used in
other projects (e.g. Oscar), so we should allow treebeard 4.0 which
is compatible with Django 1.9.

setup.py

@@ -24,7 +24,7 @@ install_requires = [
     "Django>=1.8.1,<1.10",
     "django-modelcluster>=1.1,<1.2",
     "django-taggit>=0.17.5",
-    "django-treebeard==3.0",
+    "django-treebeard>=3.0,<5.0",
     "djangorestframework>=3.1.3",
     "Pillow>=2.6.1",
     "beautifulsoup4>=4.3.2",

tox.ini

@@ -21,7 +21,7 @@ basepython =
 deps =
     django-modelcluster>=1.1b1
     django-taggit>=0.17.5
-    django-treebeard==3.0
+    django-treebeard>=3.0,<5.0
     django-sendfile==0.3.6
     djangorestframework==3.1.3
     Pillow>=2.3.0