make sprockets cache definition development-only

config/initializers/assets.rb

@@ -1,11 +1,13 @@
 # Be sure to restart your server when you modify this file.
 
-Rails.application.config.assets.configure do |env|
+unless Rails.env.production?
+  Rails.application.config.assets.configure do |env|
     env.cache = Sprockets::Cache::FileStore.new(
       ENV.fetch("SPROCKETS_CACHE", "#{env.root}/tmp/cache/assets"),
       Rails.application.config.assets.cache_limit,
       env.logger
     )
+  end
 end
 
 # Version of your assets, change this if you want to expire all your assets.