reduce time problems with VAPID expiration (thx @gauntface)

cf. web-push-libs/web-push#258
cf. #63

src/VAPID.php

@@ -104,7 +104,7 @@ public static function validate(array $vapid)
      */
     public static function getVapidHeaders($audience, $subject, $publicKey, $privateKey, $expiration = null)
     {
-        $expirationLimit = time() + 86400;
+        $expirationLimit = time() + 43200; // equal margin of error between 0 and 24h
         if (!isset($expiration) || $expiration > $expirationLimit) {
             $expiration = $expirationLimit;
         }