Since I used RVM and only had one Ruby on Rails project I was working on, doing so is easy, though not intuitive.
To uninstall all the gems:
$ rvm gemset empty
Then install them the regular way:
$ bundle install
You might get an error that looks like this: “There was an error in your Gemfile, and Bundler cannot continue.”. If that’s the case, run:
$ gem install bundler