Fix issues found by rubocop

[Rubocop](https://github.com/bbatsov/rubocop) is a useful code quality checker / linter for Ruby.

## Notes
- You will probably want to `gem install rubocop`.
- You can automatically fix most rubocop issues with `rubocop -a`.
- You should remove this from `.rubocop.yml` when the documentation is merged in so rubocop can report missing class/module doc comments:
```
Documentation:
  Enabled: false
```