Could not find gem 'annotate-models (= 1.0.4)' in any of the gem sources listed in your Gemfile.
The solution is to use the annotate gem instead:
gem 'annotate', '2.4.0'
$ bundle install
In order to reproduce the behavior of the annotate command provided by annotate-models, you should use bundle exec and pass the new annotate command the before option:
$ bundle exec annotate --position before
The Ruby on Rails Tutorial book has been updated accordingly.