Laravel 5.6 updates

Travis config update

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

vendor/symfony/http-kernel/HttpCache/Store.php

@@ -29,11 +29,9 @@ class Store implements StoreInterface
     private $locks;
 
     /**
-     * @param string $root The path to the cache directory
-     *
      * @throws \RuntimeException
      */
-    public function __construct($root)
+    public function __construct(string $root)
     {
         $this->root = $root;
         if (!file_exists($this->root) && !@mkdir($this->root, 0777, true) && !is_dir($this->root)) {