Fix error resolving relation parents

It's not entirely clear why specifying the foreign key explicitly
fixes this - the value given is what it should default to - but
without this it finds the child relation instead when looking at
the parents of a relation.

Fixes #789

app/models/relation_member.rb

@@ -2,6 +2,6 @@ class RelationMember < ActiveRecord::Base
   self.table_name = "current_relation_members"
   self.primary_keys = "relation_id", "sequence_id"
 
-  belongs_to :relation
+  belongs_to :relation, :foreign_key => :relation_id
   belongs_to :member, :polymorphic => true
 end