Fix deprecated warning on meta.get_all_field_names with Django 1.9

Author: stephane

webstack_django_sorting/templatetags/sorting_tags.py

@@ -133,7 +133,8 @@ def need_python_sorting(self, queryset, ordering):
 
         # Python sorting if not a field
         field = ordering[1:] if ordering[0] == '-' else ordering
-        return field not in queryset.model._meta.get_all_field_names()
+        field_names = [f.name for f in queryset.model._meta.get_fields()]
+        return field not in field_names
 
     def render(self, context):
         if self.context_var is not None: