Yakstrap is a pure CSS framework that was written to be mostly compatible with bootstrap 2.0 while not being bootstrap. It is missing a lot of bootstrap's more interactive elements and that's intentional, not requiring JS. Yakstrap was designed to be capable of being both responsive and flow by default, though some elements will not work on mobile systems.
Gemfile gem 'yakstrap', '~> 0.0.6'
install gem install yakstrap