Conform to Ruby style guide on %r

Prior to this change rubocop gives:

```
Guardfile:26:9: C: Use %r only for regular expressions matching more than 1 '/' character.
  watch(%r{.+\.rb$})
        ^^^^^^^^^^^
Guardfile:27:9: C: Use %r only for regular expressions matching more than 1 '/' character.
  watch(%r{(?:.+/)?\.rubocop\.yml$}) { |m| File.dirname(m[0]) }
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^
```