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.
@JakubOnderka: Looks ok:
```
$ ./parallel-lint src/
PHP 5.6.0 | 10 parallel jobs
...............                                              15/15 (100 %)
Checked 15 files in 0.3 seconds, no syntax error found

$ ./parallel-lint -p hhvm src/
PHP 5.6.99 | HHVM version 3.3.0 | 10 parallel jobs
...............                                              15/15 (100 %)
Checked 15 files in 5.2 seconds, no syntax error found
```