[#4420] Add a RetainerDeliverable model.

test/unit/retainer_deliverable_test.rb

@@ -0,0 +1,7 @@
+require File.dirname(__FILE__) + '/../test_helper'
+
+class RetainerDeliverableTest < ActiveSupport::TestCase
+  should "be a subclass of HourlyDeliverable" do
+    assert_equal HourlyDeliverable, RetainerDeliverable.superclass
+  end
+end