Laravel 5.6 updates

Travis config update

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

vendor/symfony/var-dumper/Dumper/HtmlDumper.php

@@ -56,7 +56,7 @@ class HtmlDumper extends CliDumper
     /**
      * {@inheritdoc}
      */
-    public function __construct($output = null, $charset = null, $flags = 0)
+    public function __construct($output = null, string $charset = null, int $flags = 0)
     {
         AbstractDumper::__construct($output, $charset, $flags);
         $this->dumpId = 'sf-dump-'.mt_rand();