Lint and update Ruby code

Since we now use RuboCop when creating a pull request, it is advisable to check and correct the current offenses RuboCop shows.

- [ ] Remove `.rubocop_todo.yml`
- [ ] Address issues

```bash
$ rubocop
(...)
62 files inspected, 289 offenses detected, 230 offenses autocorrectable
user=6.72s system=1.00s cpu=204% total=3.768
```