Gem 'sitemap', running rake on server?

I need to create sitemap file, so I found interesting gem 'sitemap'. I have deployed my app with capistrano. And already users have created a lot of posts.

So, I did everything gem is told me to do. I configured sitemap.rb and run rake sitemap:generate. But the I look at generated file, and it have map of my local version of app (obviously). So I loaded ready sitemap.rb file to server. But I can't run rake command on server, it giving me:

rake aborted!
Gem::LoadError: You have already activated rake 10.4.2

And I don't understand how can I run rake command on my server.