add dummy initializer

spec/dummy/config/initializers/assets.rb

@@ -0,0 +1,12 @@
+# 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'
+
+# 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 )
+Rails.application.config.assets.precompile += %w( icons/*.png )