Add rack to runtime dependency

[pocke]

I'm not sure the status of this repository. So if this pull request is not mergeable now, I think we keep open this pull request.

Rails/HttpStatus cop uses rack gem as an additional dependency to display more descriptive offense messages. The runtime dependencies do not include rack because RuboCop needs the gem only for the cop.

I think we can add rack to runtime dependencies after separating rubocop-rails gem.
rubocop is used without rack sometimes, but rubocop-rails gem will be used with rails gem in most cases, so the environment includes rack already.
If we add rack, installed gems does not increase.

[andyw8]

For reference: https://github.com/rubocop-hq/rubocop/issues/5745

[koic]

This looks useful to me.

I'm not sure the status of this repository.

This repository is WIP. I'm doing a hack like RuboCop::RSpec::Inject now.
rubocop/rubocop#5976 (comment)

I think we can release after that hack (and prepare minimal document) . When releasing, I'd like report again to @toshimaru about repository name.
toshimaru/rubocop-rails#31 (comment)

[pocke]

Thank you for your merging and describing, @koic !
Feel free to tell me if the cop has any problems such as backport.