Boo, now development is limited to c-api-compatible rubies :(

My practice is to use the gemfile for tools that I use in development but that development doesn't depend on.  e.g. I would put debugger in a Gemfile since you may use it while developing, but running tests doesn't depend on it.  That way, someone running tests, only need install the test dependencies.  (Which is how bundler interprets the gemspec command).

Would you accept a PR to put debugger in the Gemfile specifying the ruby and rbx platforms?

or see http://confreaks.com/videos/2864-rubyconf2013-repl-driven-development-with-pry :)