title: Rails with ruby-debugger throw 'Symbol not found: _ruby_current_thread (LoadError)'
body: <p>I have standard ruby-head and Rails 3.1rc4.
  I installed ruby-debug  with following instructions from <a href="http://dirk.net/2010/04/17/ruby-debug-with-ruby-19x-and-rails-3-on-rvm/">http://dirk.net/2010/04/17/ruby-debug-with-ruby-19x-and-rails-3-on-rvm/</a></p>
  
  <p>But when i run <code>rails s --debugger</code> it will throw this strange error</p>
  
  <pre><code>/Users/schovi/.rvm/gems/ruby-head/gems/activesupport-3.1.0.rc4/lib/active_support/dependencies.rb:237:in `require': dlopen(/Users/schovi/.rvm/gems/ruby-head/gems/linecache19-0.5.12/lib/trace_nums19.bundle, 9): Symbol not found: _ruby_current_thread (LoadError)
    Referenced from: /Users/schovi/.rvm/gems/ruby-head/gems/linecache19-0.5.12/lib/trace_nums19.bundle
    Expected in: flat namespace
  in /Users/schovi/.rvm/gems/ruby-head/gems/linecache19-0.5.12/lib/trace_nums19.bundle - /Users/schovi/.rvm/gems/ruby-head/gems/linecache19-0.5.12/lib/trace_nums19.bundle
  from /Users/schovi/.rvm/gems/ruby-head/gems/activesupport-3.1.0.rc4/lib/active_support/dependencies.rb:237:in `block in require'
  from /Users/schovi/.rvm/gems/ruby-head/gems/activesupport-3.1.0.rc4/lib/active_support/dependencies.rb:223:in `block in load_dependency'
  </code></pre>
  
  <p>Any idea?
  Thanks, David</p>