fix(cloud): just return a string (#363)

When I run the cloud function example under `http://localhost:1337/test` I get an error message ` {"code":141,"error":"res.success is not a function"}`. This fixes it.

Author: moklick

cloud/main.js

@@ -1,4 +1,3 @@
-
 Parse.Cloud.define('hello', function(req, res) {
-  res.success('Hi');
+  return 'Hi';
 });