Add generic types to traversable implementations

Author: derrabus

Transport/RoundRobinTransport.php

@@ -24,6 +24,9 @@
  */
 class RoundRobinTransport implements TransportInterface
 {
+    /**
+     * @var \SplObjectStorage<TransportInterface, float>
+     */
     private $deadTransports;
     private $transports = [];
     private $retryPeriod;