Add responders gem as solidus_backend dependency

It is now loaded and required through solidus_api (and
solidus_auth_devise if included) but respond_with is widely used
and it is better to explicitely require it.

backend/lib/spree/backend.rb

@@ -9,6 +9,7 @@
 require 'autoprefixer-rails'
 require 'jbuilder'
 require 'kaminari'
+require 'responders'
 
 require 'spree_core'
 require 'spree_api'

backend/solidus_backend.gemspec

@@ -29,6 +29,7 @@ Gem::Specification.new do |s|
  s.add_dependency 'jbuilder', '~> 2.8'
  s.add_dependency 'jquery-rails'
  s.add_dependency 'kaminari', '~> 1.1'
+ s.add_dependency 'responders'
  s.add_dependency 'sassc-rails'
 
  s.add_dependency 'autoprefixer-rails'