Use edge RuboCop for internal_investigation task

[koic]

This configuration reproduces internal_investigation task of rubocop-hq/rubocop repo.

[marcandre]

Thanks for checking this out.

I feel it's best not to add rubocop as a dependency to be sure that rubocop-ast is well self-containted.

Bundler setups all dependencies so it would be hard to detect if the rubocop-ast gem was actually requiring some code from the main rubocop gem. Specs would pass, but the gem wouldn't actually be self contained and anyone requiring just this gem could have their code crash...

[koic]

Ah... sure! rubocop-ast will be a widely used gem outside the RoboCop core. I misunderstood this gem as part of RuboCop 💦 Thank you for gemify and explanation!

[marcandre]

😆 Not sure it will be widely used outside of RuboCop, but the goal is that it can be used separately 😸

I will add a note in the gemspec and Gemfile.