Use byebug on Ruby 2+

The debugger gem does not seem to be properly working on Ruby 2 Show archive.org snapshot . Use byebug instead!

Byebug is a simple to use, feature rich debugger for Ruby 2. It uses the new TracePoint API for execution control and the new Debug Inspector API for call stack navigation, so it doesn't depend on internal core sources. It's developed as a C extension, so it's fast. And it has a full test suite so it's reliable. Note that byebug works only for ruby 2.0.0 or newer. For debugging ruby 1.9.3 or older, use debugger.