Fixes #400

After more digging than I expected (and planned), I'm 99% sure that the problem comes from `graphene_django`.

I found a [comment](https://github.com/django/django/blob/16436f3751e9eec67a7b80b25c8e7d29a34be67e/django/db/models/fields/reverse_related.py#L4-L6) in Django's source that refers to an attribute named `remote_field`, instead of `rel`.

It seamed to fix the problem for me without any other bugs (yet).

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.rel
+        rel = f.field.remote_field
         default = {
             'name': name,
             'label': capfirst(rel.related_name)