Register extension on Sprockets.

The `app.assets` instance is `nil` in Rails 4.1.

lib/handlebars_assets/engine.rb

@@ -2,7 +2,7 @@ module HandlebarsAssets
   # NOTE: must be an engine because we are including assets in the gem
   class Engine < ::Rails::Engine
     initializer "handlebars_assets.assets.register", :group => :all do |app|
-      ::HandlebarsAssets::register_extensions(app.assets)
+      ::HandlebarsAssets::register_extensions(Sprockets)
     end
   end
 end