There is no way to do this in Bundler.
So, you have the following options:
Do not use twitter-bootstrap-rails. You can simply copy the compiled css and js files to the appropriate directories in vendor/assets. You will lose the ability to change fewer variables. Or you can use a gem compass_twitter_bootstrapthat uses sass instead of sass.
less execjs commonjs therubyracer. , (-), .
:platform Gemfile, OSX Linux. , , less. , .