Remove treebeard (temporarily) from the default core apps.

The treebeard admin templatetags are not yet compatible with Django 1.9,
workaround issues by simply removing it from `INSTALLED_APPS`.

src/oscar/__init__.py

     'oscar.apps.dashboard.shipping',
     # 3rd-party apps that oscar depends on
     'haystack',
-    'treebeard',
+    # 'treebeard',
     'sorl.thumbnail',
     'django_tables2',
 ]

src/oscar/apps/catalogue/admin.py

 from django.contrib import admin
-from treebeard.admin import TreeAdmin
-from treebeard.forms import movenodeform_factory
 
 from oscar.core.loading import get_model
 
     inlines = [AttributeOptionInline, ]
 
 
-class CategoryAdmin(TreeAdmin):
-    form = movenodeform_factory(Category)
+class CategoryAdmin(admin.ModelAdmin):
+    list_display = ('name', 'slug')
 
 
 admin.site.register(ProductClass, ProductClassAdmin)