add Rails 4.2 gemfile and test configuration
Gemfile.rails-4-2 adds the 'responders' gem, since ActiveController::Responder has been removed from Rails 4.2

To test in Rails 4.2:
  bundle install --gemfile gemfiles/Gemfile.rails4-2
  BUNDLE_GEMFILE=gemfiles/Gemfile.rails4-2 bundle exec rake test

Currently:
81 runs, 92 assertions, 16 failures, 1 errors, 1 skips