Update acl.js

remove deprecated bluebird defer to avoid warnings

Author: xelaz

lib/acl.js

@@ -78,8 +78,6 @@ var Acl = function (backend, logger, options){
   @return {Promise} Promise resolved when finished
 */
 Acl.prototype.addUserRoles = function(userId, roles, cb){
-  var promise = bluebird.defer();
-
   contract(arguments)
     .params('string|number','string|array','function')
     .params('string|number','string|array')