Remove the "HHVM tested" badge

While this project is actually tested against HHVM, we are no longer
using the default Travis CI PHP testing configuration, so the
http://hhvm.h4cc.de/ logic can't determine whether we're testing against
HHVM. So that the badge doesn't show up red and cause confusion, I'm
removing it.

[ci skip]

README.md

@@ -7,7 +7,6 @@ _NOTICE: Formerly known as `rhumsaa/uuid`, The package and namespace names have
 [![Latest Version][badge-release]][release]
 [![Software License][badge-license]][license]
 [![Build Status][badge-build]][build]
-[![HHVM Status][badge-hhvm]][hhvm]
 [![Scrutinizer][badge-quality]][quality]
 [![Coverage Status][badge-coverage]][coverage]
 [![Total Downloads][badge-downloads]][downloads]
@@ -152,7 +151,6 @@ The ramsey/uuid library is copyright © [Ben Ramsey](https://benramsey.com/) and
 [badge-release]: https://img.shields.io/github/release/ramsey/uuid.svg?style=flat-square
 [badge-license]: https://img.shields.io/badge/license-MIT-brightgreen.svg?style=flat-square
 [badge-build]: https://img.shields.io/travis/ramsey/uuid/master.svg?style=flat-square
-[badge-hhvm]: https://img.shields.io/hhvm/ramsey/uuid.svg?style=flat-square
 [badge-quality]: https://img.shields.io/scrutinizer/g/ramsey/uuid/master.svg?style=flat-square
 [badge-coverage]: https://img.shields.io/coveralls/ramsey/uuid/master.svg?style=flat-square
 [badge-downloads]: https://img.shields.io/packagist/dt/ramsey/uuid.svg?style=flat-square
@@ -162,7 +160,6 @@ The ramsey/uuid library is copyright © [Ben Ramsey](https://benramsey.com/) and
 [release]: https://github.com/ramsey/uuid/releases
 [license]: https://github.com/ramsey/uuid/blob/master/LICENSE
 [build]: https://travis-ci.org/ramsey/uuid
-[hhvm]: http://hhvm.h4cc.de/package/ramsey/uuid
 [quality]: https://scrutinizer-ci.com/g/ramsey/uuid/
 [coverage]: https://coveralls.io/r/ramsey/uuid?branch=master
 [downloads]: https://packagist.org/packages/ramsey/uuid