Support HHVM

I ran into this when using composer to build a project on a server that uses HHVM with php54 compat. Composer satisfied the php version requirement with hhvm, but the test still failed (early on) inside PHP-Parallel-Lint. Its parent process uses `PHP_VERSION_ID` but the lint subprocesses fail due to it using `php -v`.

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Blocked on nette/tester#156 going into a release.
I'm not using the `hhvm` executable. HHVM ships with a `php` executable which starts in this mode already. Under HHVM, `php -r 'echo PHP_VERSION_ID;'` works fine.