Fixed Django 1.8 support for related field widgets.

The RelatedFieldWidgetWrapper tries to resolve the "change" url with
"__fk__" as ID value, which broke with our strict pk_regex

polymorphic/admin.py

@@ -120,7 +120,7 @@ class PolymorphicParentModelAdmin(admin.ModelAdmin):
     #: The regular expression to filter the primary key in the URL.
     #: This accepts only numbers as defensive measure against catch-all URLs.
     #: If your primary key consists of string values, update this regular expression.
-    pk_regex = '(\d+)'
+    pk_regex = '(\d+|__fk__)'
 
 
     def __init__(self, model, admin_site, *args, **kwargs):