amol@chikhaldara:~/exp/blog$ rails s
Exiting
/home/amol/.rvm/gems/ruby-1.9.3-p0/gems/async-rack-0.5.1/lib/async_rack/async_callback.rb:7:in `const_get': uninitialized constant Rack::ContentLength (NameError)
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/async-rack-0.5.1/lib/async_rack/async_callback.rb:7:in `AsyncCallback'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/async-rack-0.5.1/lib/async_rack/content_length.rb:4:in `<module:AsyncRack>'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/async-rack-0.5.1/lib/async_rack/content_length.rb:3:in `<top (required)>'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/rack-1.4.1/lib/rack/content_length.rb:6:in `<module:Rack>'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/rack-1.4.1/lib/rack/content_length.rb:3:in `<top (required)>'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/rack-1.4.1/lib/rack/handler/webrick.rb:3:in `require'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/rack-1.4.1/lib/rack/handler/webrick.rb:3:in `<top (required)>'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/rack-1.4.1/lib/rack/handler.rb:43:in `rescue in default'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/rack-1.4.1/lib/rack/handler.rb:40:in `default'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/rack-1.4.1/lib/rack/server.rb:269:in `server'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/railties-3.2.7/lib/rails/commands/server.rb:59:in `start'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/railties-3.2.7/lib/rails/commands.rb:55:in `block in <top (required)>'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/railties-3.2.7/lib/rails/commands.rb:50:in `tap'
from /home/amol/.rvm/gems/ruby-1.9.3-p0/gems/railties-3.2.7/lib/rails/commands.rb:50:in `<top (required)>'
from script/rails:6:in `require'
from script/rails:6:in `<main>'
amol@chikhaldara:~/exp/blog$
刚刚陷入困境，任何提示如何解决这个问题