can't start rails server in debug mode

I am greenhorn in the world of RoR.

When I try to start rails server in debug mode

 rails server --debug
=> Booting Thin
=> Rails 3.2.6 application starting in development on http://0.0.0.0:3000
=> Call with -d to detach
=> Ctrl-C to shutdown server
You need to install ruby-debug to run the server in debugging mode. With gems, use 'gem install ruby-debug'
Exiting

This is my ruby version

ruby -v
ruby 1.9.3p551 (2014-11-13 revision 48407) [x86_64-linux]

rails version

rails -v
Rails 3.2.6

installed debugger gems

gem list ruby-debug

*** LOCAL GEMS ***

ruby-debug-base19 (0.11.25)
ruby-debug-base19x (0.11.30.pre15)
ruby-debug19 (0.11.6)

What's the problem here? All looks good to me expect I suspect something to do with rails version