remove foreign key from learning_object attachments

db/migrate/20171205140057_remove_learning_object_attachments_foreign_key_on_cascade.rb

+class RemoveLearningObjectAttachmentsForeignKeyOnCascade < ActiveRecord::Migration[5.0]
+  def change
+    remove_foreign_key :learning_object_attachments, :learning_object_attachments
+  end
+end