Upgrade RuboCop

* upgrade to the latest `0.24.1` version
* memoize `StyleGuide#config` result
* add tests for old-style syntax
* call `add_missing_namespaces` on override (custom) config to ensure that
  merging is handled properly
* disable `EndOfLine` cop in tests

https://trello.com/c/K7dSqe3k
Should we lock it down just to be safe?