Use Config#[] instead of removed Config#fetch
Use Config#[] instead of removed Config#fetch

In RuboCop v0.43.0, the `Config` class no longer inherits from `Hash`, and thus
no longer implements all of `Hash`'s API.

andyw8/rubocop-rspec,andyw8/rubocop-rspec,backus/rubocop-rspec,backus/rubocop-rspec,bquorning/rubocop-rspec,nevir/rubocop-rspec