Revert "Ruby 1.9: use ruby-debug19. [#3949 state:resolved]"

This wasn't the issue here; we need to beef up the docs and speak to
Wayne and Mark about installation.

This reverts commit da238daf.

railties/lib/rails/rack/debugger.rb

@@ -8,7 +8,7 @@ def initialize(app)
 
         ARGV.clear # clear ARGV so that rails server options aren't passed to IRB
 
-        require_library_or_gem(RUBY_VERSION < '1.9' ? 'ruby-debug' : 'ruby-debug19')
+        require_library_or_gem 'ruby-debug'
         ::Debugger.start
         ::Debugger.settings[:autoeval] = true if ::Debugger.respond_to?(:settings)
         puts "=> Debugger enabled"