Laravel 5.6 updates

Travis config update

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

vendor/symfony/http-foundation/Session/Storage/MetadataBag.php

@@ -57,7 +57,7 @@ class MetadataBag implements SessionBagInterface
      * @param string $storageKey      The key used to store bag in the session
      * @param int    $updateThreshold The time to wait between two UPDATED updates
      */
-    public function __construct($storageKey = '_sf2_meta', $updateThreshold = 0)
+    public function __construct(string $storageKey = '_sf2_meta', int $updateThreshold = 0)
     {
         $this->storageKey = $storageKey;
         $this->updateThreshold = $updateThreshold;