use cluster.on('fork') for adding forks

test.js

@@ -11,11 +11,13 @@ if (cluster.isMaster) {
   cstore = require('./master').create({
     name: 'foo-level'
   });
-  cstore.addWorker(cluster.fork());
   cstore.then(function (db) {
     db.set('foo', 'bar');
   });
 
+  cluster.fork();
+  cluster.fork();
+
 
 }
 else {