Use assets configure block

Author: josh

test/fixtures/scss_project/config/initializers/postprocessor.rb

@@ -1,3 +1,5 @@
-Rails.application.assets.register_postprocessor 'text/css', :postprocessor do |context, css|
-  css.gsub /@import/, 'fail engine'
+Rails.application.config.assets.configure do |env|
+  env.register_postprocessor 'text/css', :postprocessor do |context, css|
+    css.gsub /@import/, 'fail engine'
+  end
 end