Add _.pluck to backbone.html in anticipation of Backbone's version bump. [ci skip]

test/backbone.html

@@ -31,7 +31,8 @@
 		<script>
 			_.mixin({
 				'debounce': _.debounce || lodash.debounce,
-				'defer': _.defer || lodash.defer
+				'defer': _.defer || lodash.defer,
+				'pluck': _.pluck || lodash.pluck
 			});
 		</script>
 		<script src="../vendor/backbone/backbone.js"></script>