Laravel 5.6 updates

Travis config update

Removed HHVM script as Laravel no longer support HHVM after releasing 5.3

vendor/symfony/http-kernel/Event/KernelEvent.php

@@ -32,7 +32,7 @@ class KernelEvent extends Event
      * @param int                 $requestType The request type the kernel is currently processing; one of
      *                                         HttpKernelInterface::MASTER_REQUEST or HttpKernelInterface::SUB_REQUEST
      */
-    public function __construct(HttpKernelInterface $kernel, Request $request, $requestType)
+    public function __construct(HttpKernelInterface $kernel, Request $request, ?int $requestType)
     {
         $this->kernel = $kernel;
         $this->request = $request;