Change chain_driver service to public

[nmpolo]

This bundle currently doesn't work with Symfony 3.4 and 4.x as it throws the following error:

The "trappar_alice_generator.metadata.chain_driver" service or alias has been removed or inlined when the container was compiled. You should either make it public, or stop using the container directly and use dependency injection instead.

Fixes https://github.com/trappar/AliceGeneratorBundle/issues/7