Return the deferred in auth.logout().

Refs: #7992

@@ -250,7 +250,7 @@ define(function(require) {
             };
             this.logout = function() {
                 _.invoke(this.get('providers').values(), 'logout');
-                api('auth:logout');
+                return api('auth:logout');
             };
             bindMethods(this, 'login', 'logout');
         },