After updating a bundle, you will have some gems that may be obsolete -- since a newer version of that gem has been installed. There is no such command under the bundle executable i.e. bundle clean. How does one get rid of these obsolete gems?
This is an attempt to reduce slug size in my rails app.
If you are using Bundler 1.1 or later you can use bundle clean, just as you imagined you could. This is redundant if you're using bundle install --path (Bundler manages the location you specified with --path, so takes responsibility for removing outdated gems), but if you've used Bundler to install the gems as system gems then bundle clean --force will delete any system gems not required by your Gemfile. Blindingly obvious caveat: don't do this if you have other apps that rely on system gems that aren't in your Gemfile!
Pat Shaughnessy has a good description of bundle clean and other new additions in bundler 1.1.