style: use RuboCop 0.41.2, adjust defaults (#489)

Changes to our style configuration:

- Consolidate all rules related to Ruby 1.8 compatibility in one place.
- Codify our de-facto preference for `alias_method` over `alias` (drops
  offense count by 54 after turning this on).
- Drop `Style/SignalException` as `only_raise` has been the new default
  for quite a while (since RuboCop 0.37.0).