rack can't load file (LoadError)

gistfile1.txt

- browser output:

cannot load such file -- rack (LoadError)
  /var/lib/gems/1.9.1/gems/passenger-4.0.20/helper-scripts/rack-preloader.rb:100:in `require'
  /var/lib/gems/1.9.1/gems/passenger-4.0.20/helper-scripts/rack-preloader.rb:100:in `preload_app'
  /var/lib/gems/1.9.1/gems/passenger-4.0.20/helper-scripts/rack-preloader.rb:150:in `<module:App>'
  /var/lib/gems/1.9.1/gems/passenger-4.0.20/helper-scripts/rack-preloader.rb:29:in `<module:PhusionPassenger>'
  /var/lib/gems/1.9.1/gems/passenger-4.0.20/helper-scripts/rack-preloader.rb:28:in `<main>'


- Log file
[ 2015-06-11 02:14:14.9007 32594/7f9a34265700 Pool2/Spawner.h:738 ]: [App 347 stdout] 
[ 2015-06-11 02:14:15.2296 32594/7f9a34265700 Pool2/Implementation.cpp:849 ]: Could not spawn process for group /var/www/apps/tcj/current#default: An error occured while starting up the preloader.
     in 'void Passenger::ApplicationPool2::SmartSpawner::handleErrorResponse(Passenger::ApplicationPool2::SmartSpawner::StartupDetails&)' (SmartSpawner.h:455)
     in 'std::string Passenger::ApplicationPool2::SmartSpawner::negotiatePreloaderStartup(Passenger::ApplicationPool2::SmartSpawner::StartupDetails&)' (SmartSpawner.h:558)
     in 'void Passenger::ApplicationPool2::SmartSpawner::startPreloader()' (SmartSpawner.h:206)
     in 'virtual Passenger::ApplicationPool2::ProcessPtr Passenger::ApplicationPool2::SmartSpawner::spawn(const Passenger::ApplicationPool2::Options&)' (SmartSpawner.h:744)
     in 'void Passenger::ApplicationPool2::Group::spawnThreadRealMain(const SpawnerPtr&, const Passenger::ApplicationPool2::Options&, unsigned int)' (Implementation.cpp:782)

[ 2015-06-11 02:14:15.2297 32594/7f9a32000700 agents/HelperAgent/RequestHandler.h:1975 ]: [Client 20] Cannot checkout session. An error occured while starting up the preloader.
Error page:
cannot load such file -- rack (LoadError)
  /var/lib/gems/1.9.1/gems/passenger-4.0.20/helper-scripts/rack-preloader.rb:100:in `require'
  /var/lib/gems/1.9.1/gems/passenger-4.0.20/helper-scripts/rack-preloader.rb:100:in `preload_app'
  /var/lib/gems/1.9.1/gems/passenger-4.0.20/helper-scripts/rack-preloader.rb:150:in `<module:App>'
  /var/lib/gems/1.9.1/gems/passenger-4.0.20/helper-scripts/rack-preloader.rb:29:in `<module:PhusionPassenger>'
  /var/lib/gems/1.9.1/gems/passenger-4.0.20/helper-scripts/rack-preloader.rb:28:in `<main>'