Ideally, public-suffix would be automatically installed when you’re installing jekyll for the first time because public-suffix is a dependency of addressable which is in turn a dependency of jekyll…
The sane way out here is to manually install public_suffix:
gem install public_suffix --version 3.0.1
Alternatively, if you’re working on a fork of a Jekyll site / theme repo that already has a Gemfile, you can simply run:
bundle update