Laravel 5.6 updates

Travis config update

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

vendor/symfony/http-kernel/UriSigner.php

@@ -25,7 +25,7 @@ class UriSigner
      * @param string $secret    A secret
      * @param string $parameter Query string parameter to use
      */
-    public function __construct($secret, $parameter = '_hash')
+    public function __construct(string $secret, string $parameter = '_hash')
     {
         $this->secret = $secret;
         $this->parameter = $parameter;