Webrick Server don't start after Update Rails

After a Rails update (rails-1.1.6) , Webrick server don't start.

  Cannot find gem for Rails =1.1.4:


  -> How can I best solve the problem for the old projects:
          Starting a new project and copy the files?

Go to your application config/ folder and edit environment.rb.
There should be line RAILS_GEM_VERSION = '1.1.4'.
Change it to match current Rails version.