Fix false positives for readonly classes (fix #213)

[daniil-berg]

See #213.

Also, this discussion may be relevant.

For now, since PHPCSUtils is in the dependency tree of moodle-cs (required by phpcsstandards/phpcsextra), this should solve the issue.

EDIT: It looks like the former will become even more closely tied to PHPCS (see here), so all the more reason to use it here explicitly.

I also took the liberty of improving readability a bit by doing $token = $tokens[$i]; at the start of the loop. Hope this is OK.