update gemspec to use binding.repl v3.0.0

Hi,
binding.repl v3.0.0 was released today. The pull request updates the gemspec to depend on ~> 3.0.0, and updates irbtools to use `BindingRepl.auto =` instead of `Binding.repl.auto_order =` which was removed and renamed in v3.0.0 because I didn't think it was worth the monkey patch on the Binding class. I also updated the link in the README file since it currently 404s. 

I have prepared a pull request for the 'debugging' gem since its dependency currently conflicts with the dependency that irbtools has in this PR. v3.0.0 of binding.repl removes some poor decisions I made when I started the project, and it also supports the lesser known "ir" repl. I hope you can accept the PR. Let me know if there is any changes you'd like me to make before you can merge it. Thanks!