Although having both debugger and byebug installed at the same time _might_ work, it looks like a source of conflicts. I would do something like this instead
    gem "debugger", :platforms => [:mingw_19, :ruby_19]
    gem 'byebug', :platforms => [:mingw_20, :ruby_20]
    gem 'pry-byebug', :platforms => [:mingw_20, :ruby_20]