getting error in passenger

cannot load such file -- rack (LoadError)
  /usr/local/rvm/rubies/ruby-2.1.1/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:54:in `require'
  /usr/local/rvm/rubies/ruby-2.1.1/lib/ruby/site_ruby/2.1.0/rubygems/core_ext/kernel_require.rb:54:in `require'
  /usr/local/rvm/gems/ruby-2.2.1/gems/passenger-5.0.14/lib/phusion_passenger/loader_shared_helpers.rb:395:in `activate_gem'
  /usr/local/rvm/gems/ruby-2.2.1/gems/passenger-5.0.14/helper-scripts/rack-preloader.rb:101:in `preload_app'
  /usr/local/rvm/gems/ruby-2.2.1/gems/passenger-5.0.14/helper-scripts/rack-preloader.rb:153:in `<module:App>'
  /usr/local/rvm/gems/ruby-2.2.1/gems/passenger-5.0.14/helper-scripts/rack-preloader.rb:29:in `<module:PhusionPassenger>'
  /usr/local/rvm/gems/ruby-2.2.1/gems/passenger-5.0.14/helper-scripts/rack-preloader.rb:28:in `<main>'