Added reference to Underscore

backbone-relational/backbone-relational-tests.ts

@@ -1,4 +1,5 @@
 /// <reference path="../backbone/backbone.d.ts" />
+/// <reference path="../underscore/underscore.d.ts" />
 /// <reference path="backbone-relational.d.ts" />
 
 class House extends Backbone.RelationalModel {