Downgrade RuboCop to avoid `bundle install` error with Ruby 2.4
Downgrade RuboCop to avoid `bundle install` error with Ruby 2.4

```
Bundler could not find compatible versions for gem "parser":
  In Gemfile:
    mutant-rspec was resolved to 0.5.0, which depends on
      mutant (~> 0.5.0) was resolved to 0.5.1, which depends on
        parser (~> 2.1.4)

    rubocop (= 1.12.0) was resolved to 1.12.0, which depends on
      parser (>= 3.0.0.0)
```

sickill/rainbow