How to integrate payu money payment gateway in rails 4

How to integrate Payu Money in rails 4.

I have not found any proper documentation.

I use this gem http://ift.tt/1Pi6cwl

but in this i got some error when i run 'rails s'.

ERROR :

/home/tps/.rvm/gems/ruby-2.2.0/gems/active_merchant_payu_in-0.0.1/lib/active_merchant_payu_in.rb:2:in `require': cannot load such file -- active_merchant/billing/integrations (LoadError)