Name error ,uninitialized constant TurboDevAssets (NameError)

I'm learning rails.I download the code from http://ift.tt/fJLONu but it can't run.It's the error codes:

 /home/a1/.rvm/rubies/ruby-          2.1.7/web/diaspora/config/environments/development.rb:42:in `block in <top (required)>': uninitialized constant TurboDevAssets (NameError)
    from /home/a1/.rvm/gems/ruby-2.1.7/gems/railties-4.2.6/lib/rails/railtie.rb:210:in `instance_eval'
    from /home/a1/.rvm/gems/ruby-2.1.7/gems/railties-4.2.6/lib/rails/railtie.rb:210:in `configure'
    from /home/a1/.rvm/gems/ruby-2.1.7/gems/railties-4.2.6/lib/rails/railtie.rb:182:in `configure'
    from /home/a1/.rvm/rubies/ruby-2.1.7/web/diaspora/config/environments/development.rb:1:in `<top (required)>'

the config/environments/development.rb is

    Diaspora::Application.configure do
     ....
      # Speed up asset serving
      config.middleware.insert 0, TurboDevAssets
    end

how to resolve the problem?