uninitialized constant Rack::Throttle::Limiter (NameError) #38

gem 'rack-throttle', '~> 0.4.2'

when i run i am getting this error, can you please provide solution for this

/data/project/vendor/bundle/ruby/2.2.0/gems/rack-throttle-0.4.2/lib/rack/throttle/time_window.rb:3:in <module:Throttle>': uninitialized constant Rack::Throttle::Limiter (NameError) from /data/project/vendor/bundle/ruby/2.2.0/gems/rack-throttle-0.4.2/lib/rack/throttle/time_window.rb:1:in module:Rack'
from /data/project/vendor/bundle/ruby/2.2.0/gems/rack-throttle-0.4.2/lib/rack/throttle/time_window.rb:1:in <top (required)>' from /data/project/vendor/bundle/ruby/2.2.0/gems/backports-3.6.8/lib/backports/std_lib.rb:9:in require'
from /data/project/vendor/bundle/ruby/2.2.0/gems/backports-3.6.8/lib/backports/std_lib.rb:9:in require_with_backports' from /data/project/lib/app/rate_limiter/request_throttle.rb:1:in <top (required)>'
from /data/project/vendor/bundle/ruby/2.2.0/gems/backports-3.6.8/lib/backports/std_lib.rb:9:in require' from /data/project/vendor/bundle/ruby/2.2.0/gems/backports-3.6.8/lib/backports/std_lib.rb:9:in require_with_backports'
from config.ru:11:in block in <main>' from /data/project/vendor/bundle/ruby/2.2.0/gems/rack-1.5.5/lib/rack/builder.rb:55:in instance_eval'
from /data/project/vendor/bundle/ruby/2.2.0/gems/rack-1.5.5/lib/rack/builder.rb:55:in initialize' from config.ru:1:in new'
from config.ru:1:in <main>' from /data/project/vendor/bundle/ruby/2.2.0/gems/unicorn-4.6.3/lib/unicorn.rb:48:in eval'
from /data/project/vendor/bundle/ruby/2.2.0/gems/unicorn-4.6.3/lib/unicorn.rb:48:in block in builder' from /data/project/vendor/bundle/ruby/2.2.0/gems/unicorn-4.6.3/lib/unicorn/http_server.rb:722:in call'
from /data/project/vendor/bundle/ruby/2.2.0/gems/unicorn-4.6.3/lib/unicorn/http_server.rb:722:in build_app!' from /data/project/vendor/bundle/ruby/2.2.0/gems/unicorn-4.6.3/lib/unicorn/http_server.rb:140:in start'
from /data/project/vendor/bundle/ruby/2.2.0/gems/unicorn-4.6.3/bin/unicorn:126:in <top (required)>' from /data/project/vendor/bundle/ruby/2.2.0/bin/unicorn:23:in load'
from /data/project/vendor/bundle/ruby/2.2.0/bin/unicorn:23:in `