Laravel 5.6 updates

Travis config update

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

vendor/symfony/var-dumper/Caster/ClassStub.php

@@ -22,7 +22,7 @@ class ClassStub extends ConstStub
      * @param string   A PHP identifier, e.g. a class, method, interface, etc. name
      * @param callable The callable targeted by the identifier when it is ambiguous or not a real PHP identifier
      */
-    public function __construct($identifier, $callable = null)
+    public function __construct(string $identifier, $callable = null)
     {
         $this->value = $identifier;