Laravel 5.6 updates

Travis config update

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

vendor/symfony/finder/Comparator/NumberComparator.php

@@ -39,7 +39,7 @@ class NumberComparator extends Comparator
      *
      * @throws \InvalidArgumentException If the test is not understood
      */
-    public function __construct($test)
+    public function __construct(?string $test)
     {
         if (!preg_match('#^\s*(==|!=|[<>]=?)?\s*([0-9\.]+)\s*([kmg]i?)?\s*$#i', $test, $matches)) {
             throw new \InvalidArgumentException(sprintf('Don\'t understand "%s" as a number test.', $test));