The original Jasny Bootstrap extension can be found here: http://jasny.github.io/bootstrap/. This is only a gem containing a release version for easier use with Ruby on Rails.
Add into your Gemfile
gem 'jasny-bootstrap-rails'
You also need to require bootstrap. As there are different packages (sass/less) there is no fixed requirement:
gem 'twitter-bootstrap-rails'
or
gem 'bootstrap-sass'
or
...
In your Rails directory run
bundle install
Add into your application.js
//= require jasny-bootstrap.min
Add into your application.css
*= require jasny-bootstrap.min
There are some alternatives to this Gem out in the wild but i havent found any close to up to date.