Add missing static on Debugger::output()

This method should have been static the whole time, and the lack of
static was causing tests to fail on PHP 5.6

lib/Cake/Utility/Debugger.php

@@ -698,7 +698,7 @@ class Debugger {
  * @deprecated Use Debugger::outputAs() and Debugger::addFormat(). Will be removed
  *   in 3.0
  */
-	public function output($format = null, $strings = array()) {
+	public static function output($format = null, $strings = array()) {
 		$self = Debugger::getInstance();
 		$data = null;