ActionArgs has several Bundler Gemfiles, each of which bundles different version of Rails.
% ls gemfiles/*.gemfile
gemfiles/rails_41.gemfile gemfiles/rails_42.gemfile rails_edge.gemfile
Via BUNDLE_GEMFILE ENV variable, you can tell Bundler which version of Rails to bundle.
$ BUNDLE_GEMFILE=gemfiles/rails_42.gemfile bundle ex rake test
Simply executing this command would probably run all tests against each version of Rails:
% rake test:all