Laravel 5.6 updates

Travis config update

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

vendor/symfony/polyfill-php72/README.md

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+Symfony Polyfill / Php72
+========================
+
+This component provides functions added to PHP 7.2 core:
+
+- [`spl_object_id`](https://php.net/spl_object_id)
+- [`stream_isatty`](https://php.net/stream_isatty)
+
+On Windows only:
+
+- [`sapi_windows_vt100_support`](https://php.net/sapi_windows_vt100_support)
+
+Moved to core since 7.2 (was in the optional XML extension earlier):
+
+- [`utf8_encode`](https://php.net/utf8_encode)
+- [`utf8_decode`](https://php.net/utf8_decode)
+
+Also, it provides a constant added to PHP 7.2:
+- [`PHP_OS_FAMILY`](http://php.net/manual/en/reserved.constants.php#constant.php-os-family)
+
+More information can be found in the
+[main Polyfill README](https://github.com/symfony/polyfill/blob/master/README.md).
+
+License
+=======
+
+This library is released under the [MIT license](LICENSE).