Fixing installation error.

Closes #3422611

src/EventSubscriber/NitropackSubscriber.php

@@ -9,7 +9,7 @@ use Symfony\Component\HttpKernel\Event\RequestEvent;
 use Symfony\Component\HttpKernel\KernelEvents;
 use Psr\Log\LoggerInterface;
 use Drupal\Core\Logger\RfcLogLevel;
-use Symfony\Component\HttpKernel\Event\FilterResponseEvent; 
+use Symfony\Component\HttpKernel\Event\ResponseEvent;
 
 
 /**
@@ -128,11 +128,11 @@ final class NitropackSubscriber implements EventSubscriberInterface {
   /**
    * Adds Surrogate-Key header to cacheable master responses.
    *
-   * @param \Symfony\Component\HttpKernel\Event\FilterResponseEvent $event
+   * @param \Symfony\Component\HttpKernel\Event\ResponseEvent $event
    *   The event to process.
    */
-  public function onRespond(FilterResponseEvent $event) {
-    if (!$event->isMasterRequest()) {
+  public function onRespond(ResponseEvent $event) {
+    if (!$event->isMainRequest()) {
       return;
     }