responders

Rails 4.2 removed respond_to and respond_with to the responders gem, you need to add gem 'responders', '~> 2.0' to your Gemfile to make them work.

# Gemfile
gem 'responders', '~> 2.0'

Reference

• Ruby on Rails 4.2 Release Notes - 3.2 respond_with / Class-Level respond_to