Run tests for all installed versions of ruby under rbenv

NOTES.md

This took a while for me to work out, but essentially you need to remember two things:

1. everything that needs to be prefixed with rbenv exec
2. and everything that can be needs to be prefixed with bundle exec as well

The downside to this is that you need to add rake to your Gemfile. Just running rbenv exec rake will not work, for some reason.

Rakefile

desc 'Run tests on for all versions of ruby'
task :test_all do  
  `rbenv versions --bare`.split("\n").each do |vers|
    run_for vers, ['bundle install', 'bundle exec rake']
  end
end

def run_for(vers, commands)
  system "RBENV_VERSION='#{vers}' sh -c '#{commands.map {|i| "rbenv exec #{i}"}.join(' && ')}'"
end