Support React v0.3 and PHP 5.3

@clue: I took a look at [PHPUnit_Util_PHP_Default](https://github.com/sebastianbergmann/phpunit/blob/master/src/Util/PHP/Default.php), which is used to launch tests in their own processes. It utilizes [SebastianBergmann\Environment\Runtime](https://github.com/sebastianbergmann/environment/blob/master/src/Runtime.php) to do the PHP binary detection, which is much more robust for varying platforms and interpreters (e.g. older PHP versions, HHVM).

Can you replace this `cmd()` function with `getPhpBinary()` and then conditionally use `SebastianBergmann\Environment\Runtime` if the class exists (it will for more recent PHPUnit versions). Otherwise, we can fall back to `PHP_BINARY` if defined and then finally `php`.

In the existing test functions, you can leave the `$cmd = ...` lines as-is, and just replace `PHP_BINARY` with `$this->getPhpBinary()`.