Untrack config.js AND config.json

The actual config filename shouldn't be tracked, as it prevents keeping
the project up to date with regular git pulls (uncommitted changes in
working tree).

See [bca59d4#commitcomment-7840958](https://github.com/ircanywhere/ircanywhere/commit/bca59d4f0d2fde3e6361db6406429dd351d37cde#commitcomment-7840958)