Remove conflict with HHVM

[sanmai]

GitHub Actions' VMs include HHVM even if you not using it (among many other things), therefore having a conflict with HHVM leads to a need to install a package with --ignore-platform-req=hhvm to test with it on GitHub because Composer considers HHVM even if you run Composer itself with plain old PHP. I would understand if it would consider HHVM if you would be using it with HHVM, but it isn't. (But on the other hand this makes sense since, IIRC, you can't run Composer with HHVM.)

Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - php-cs-fixer/phpunit-constraint-xmlmatchesxsd v1.2.1 requires phpunitgoodpractices/polyfill ^1.4 -> satisfiable by phpunitgoodpractices/polyfill[v1.4.0].
    - php-cs-fixer/phpunit-constraint-xmlmatchesxsd v1.2.1 requires phpunitgoodpractices/polyfill ^1.4 -> satisfiable by phpunitgoodpractices/polyfill[v1.4.0].
    - phpunitgoodpractices/polyfill v1.4.0 conflicts with hhvm[99.999].
    - hhvm 99.999 conflicts with phpunitgoodpractices/polyfill[v1.4.0].
    - Installation request for hhvm 99.999 -> satisfiable by hhvm[99.999].
    - Installation request for php-cs-fixer/phpunit-constraint-xmlmatchesxsd ^1.2.1 -> satisfiable by php-cs-fixer/phpunit-constraint-xmlmatchesxsd[v1.2.1].

Which is all downright annoying. Who's actually using HHVM outside of Facebook these days?.. Even they are not.

[keradus]

Hey. thanks for the PR. I actually done it in plenty other repos already. good catch!