Move sprockets-rails dependency to backend

Core itself doesn't care about the presence of sprockets. Backend
depends on handlebars_assets and sassc-rails, which both depend on
sprockets.

Author: jarednorman

backend/solidus_backend.gemspec

@@ -31,7 +31,8 @@ Gem::Specification.new do |s|
   s.add_dependency 'jquery-rails'
   s.add_dependency 'kaminari', '~> 1.1'
   s.add_dependency 'responders'
-  s.add_dependency 'sassc-rails'
 
+  s.add_dependency 'sprockets-rails', '!= 3.5.0'
+  s.add_dependency 'sassc-rails'
   s.add_dependency 'handlebars_assets', '~> 0.23'
 end

core/solidus_core.gemspec

@@ -47,7 +47,6 @@ Gem::Specification.new do |s|
   s.add_dependency 'kt-paperclip', ['>= 6.3', '< 8']
   s.add_dependency 'psych', ['>= 4.0.1', '< 6.0']
   s.add_dependency 'ransack', ['~> 4.0', '< 5']
-  s.add_dependency 'sprockets-rails', '!= 3.5.0'
   s.add_dependency 'state_machines', ['~> 0.6', '< 0.10.0']
   s.add_dependency 'state_machines-activerecord', ['~> 0.6', '< 0.10.0']
   s.add_dependency 'omnes', '~> 0.2.2'