Remove deprecated active_support/per_thread_registry usage

lib/app.rb

@@ -56,7 +56,6 @@ class App < Sinatra::Application
 
     SpritesCLI.new.invoke(:generate, [], :disable_optimization => true)
 
-    require 'active_support/per_thread_registry'
     require 'active_support/cache'
     sprockets.cache = ActiveSupport::Cache.lookup_store :file_store, root.join('tmp', 'cache', 'assets', environment.to_s)
   end