Make sure AccessToken extends from PersistedModel

lib/models/access-token.js

@@ -2,8 +2,7 @@
  * Module Dependencies.
  */
 
-var Model = require('../loopback').Model
-  , loopback = require('../loopback')
+var loopback = require('../loopback')
   , assert = require('assert')
   , crypto = require('crypto')
   , uid = require('uid2')
@@ -37,10 +36,11 @@ var properties = {
  * @property {Date} created When the token was created
  *  
  * @class
- * @inherits {Model}
+ * @inherits {PersistedModel}
  */
 
-var AccessToken = module.exports = Model.extend('AccessToken', properties, {
+var AccessToken = module.exports =
+  loopback.PersistedModel.extend('AccessToken', properties, {
   acls: [
     {
       principalType: ACL.ROLE,