Missing "script/runner", undocumented dependency on rails?

When I try to use this gem in a plain old ruby project, runner tasks fail with:

bundler: command not found: script/runner
Install missing gem executables with `bundle install`

I know this is exists in rails projects, but the readme didn't make it seem like Rails was a hard requirement. I'm just trying to run a script on a schedule and this gem seemed perfect.

schedule.rb:

set :output, "./checker.log"
every 1.minutes do
  runner "Checker.run"
end

I installed whenever using the instructions, added to my Gemfile and ran bundle exec wheneverize ., then whenever --update-crontab

Start with the README's runner documentation and the schedule.rb example, then inspect the script/runner command produced by whenever --update-crontab. Compare the documented setup with a plain Ruby project and the reported Bundler failure. Done should make the Rails requirement and supported runner setup explicit.