Don't take curl callback by ref

Changing these does nothing to the caller, why would you take them by ref? HHVM doesn't promote this to refs if they can't be modified: facebook/hhvm#2587

S3.php

@@ -2307,7 +2307,7 @@ private function __dnsBucketName($bucket)
 	* @param string &$data Data
 	* @return integer
 	*/
-	private function __responseHeaderCallback(&$curl, &$data)
+	private function __responseHeaderCallback($curl, $data)
 	{
 		if (($strlen = strlen($data)) <= 2) return $strlen;
 		if (substr($data, 0, 4) == 'HTTP')