HHVM support & compatibility v2

This is lighter solution of #104, without introducing arguments API etc.

ping @milo for review
Personally, I would name it `PhpInterpreter`. I know starting `I` is Nette standard but in Tester it is not used.
But mainly `Interpreter`. It is not so obvious from class name but in code, imho, it is more descriptive:
```php
$this->executable
$runner->getExecutable()
# vs.
$this->interpreter
$runner->getInterpreter()
```