If you’re running into this error when attempting to install twitter-bootstrap-rails on Rails 4.2.x:
undefined method `register_preprocessor’
Note that there is currently a bug between less-rails and the 3.x version of sprockets-rails. To resolve this issue, update your gemfile with:
gem 'sprockets-rails', '<=2.3.3'
bundle update sprockets-rails
After which, you should be good to go.