Upgrade RuboCop

* 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

https://trello.com/c/K7dSqe3k
Disable EndOfLine cop in tests

This cop breaks when dealing with strings (instead of files).

Here's a RuboCop PR to address the issue:
https://github.com/bbatsov/rubocop/pull/1203