Unfortunately due to the new interface etc it was not possible to keep things working with both cases. So Symfony 2.2+ and MonologBundle 2.2+ are required to use Monolog 1.4+.
That said, it seems to be a bug in the bundle that it requires the PSR LoggerInterface, because the old interface of Symfony is also compatible. I just pushed a v2.0.1 of the bundle, if you'd like to update and see if that works (you can rollback the changes to the requirements of monolog).