If you're wanting to get Bootstrap 3.3.6 installed alongside a Rails 4.2.5 app then http://stackoverflow.com/a/20661571/89783 has the solution.
- Copy bootstrap/dist/css/bootstrap.css to app/assets/stylesheets
- Add '*= require bootstrap' to app/assets/stylesheets/application.css
And while you're at it https://gist.github.com/suryart/7418454 has the details on how to nicely integrate flash messages using Bootstrap styling. One of the comments suggests changing flash_type to flash_type.to_sym