Laravel 5.6 updates

Travis config update

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

vendor/symfony/http-foundation/File/Exception/FileNotFoundException.php

@@ -21,7 +21,7 @@ class FileNotFoundException extends FileException
     /**
      * @param string $path The path to the file that was not found
      */
-    public function __construct($path)
+    public function __construct(string $path)
     {
         parent::__construct(sprintf('The file "%s" does not exist', $path));
     }