fix responders install for ruby 2.3

Author: senid231

Gemfile

@@ -18,6 +18,8 @@ gem 'yard', require: false
 
 gem 'activeadmin', ENV.fetch('ACTIVE_ADMIN_VERSION', '~> 2.0'), require: false
 gem 'rails', ENV.fetch('RAILS_VERSION', '6.0.0')
+# responders 3 drops ruby 2.3 support
+gem 'responders', '~> 2.4', platforms: :ruby_23
 gem 'sassc-rails', '2.1.2'
 gem 'sprockets', '3.7.2'
 gem 'sqlite3', '1.4.1'