bump binding.repl version to ~> 3.0.0

Hi,
This pull request updates the binding.repl dependency to use ~> 3.0.0. I also updated the code to favor `binding.repl!` over `binding.repl.auto`. This PR should be merged and released before https://github.com/janlelis/irbtools/pull/29 to avoid version conflicts on install, since both projects depend on different versions of binding.repl.