fix php 5.3 incompatibility

Author: robocoder

lib/WebDriver/Session.php

@@ -498,10 +498,10 @@ private function serializeArguments(array $arguments)
                 // preferably we want to detect W3C support and never set nor parse
                 // LEGACY_ELEMENT_ID, until detection is implemented, serialize to both
                 // variants, tested with Selenium v2.53.1 and v3.141.59
-                $arguments[$key] = [
+                $arguments[$key] = array(
                     Container::WEBDRIVER_ELEMENT_ID => $value->getID(),
                     Container::LEGACY_ELEMENT_ID => $value->getID(),
-                ];
+                );
                 continue;
             }