Added support for Django 1.8.x

Since I didn't know why the `rel` attribute wasn't there in the first place, maybe it's better to use the `remote_field` only if it's there and leave `rel` as a default (i.e else).

Author: eyal0803

graphene_django/filter/filterset.py

@@ -57,7 +57,7 @@ def filter_for_reverse_field(cls, f, name):
         Global IDs (the default implementation expects database
         primary keys)
         """
-        rel = f.field.remote_field
+        rel = f.field.remote_field if hasattr(f.field, 'remote_field') else f.field.rel
         default = {
             'name': name,
             'label': capfirst(rel.related_name)