Push Rack-Jekyll app to Heroku

I haven't used Ruby. I don't know what gems are. But I know how to install them and do basic things like that.

http://github.com/bry4n/rack-jekyll#readme . But when I push it to Heroku, I get an error:

---> Heroku received push
---> Rack app detected
---> Installing rack-jekyll from http://rubygems.org
     ERROR: could not find gem "rack-jekyll" locally or in a repository