Add public keyword to methods

[MAILPOET-2413]

tests/integration/Cron/Workers/KeyCheck/KeyCheckWorkerMockImplementation.php

@@ -5,7 +5,7 @@ namespace MailPoet\Cron\Workers\KeyCheck;
 class KeyCheckWorkerMockImplementation extends KeyCheckWorker {
   const TASK_TYPE = 'mock_key_check_worker';
 
-  function checkKey() {
+  public function checkKey() {
     return ['code' => 12345]; // bogus code
   }
 }