Fix sassc bug with sprockets v4 in dummy app

rails/sprockets#581 (comment)

Author: tvdeyen

spec/dummy/config/initializers/assets.rb

@@ -1,11 +1,17 @@
 # Be sure to restart your server when you modify this file.
 
 # Version of your assets, change this if you want to expire all your assets.
-Rails.application.config.assets.version = '1.0'
+Rails.application.config.assets.version = "1.0"
 
 # Add additional assets to the asset load path
 # Rails.application.config.assets.paths << Emoji.images_path
 
 # Precompile additional assets.
 # application.js, application.css, and all non-JS/CSS in app/assets folder are already added.
 # Rails.application.config.assets.precompile += %w( search.js )
+
+# Fixes a bug with sprockets v4
+# https://github.com/rails/sprockets/issues/581#issuecomment-486984663
+Rails.application.config.assets.configure do |env|
+  env.export_concurrent = false
+end