setup sprockets after config parse

lib/middleman/core_extensions/sprockets.rb

@@ -5,9 +5,11 @@ module Middleman::CoreExtensions::Sprockets
     def registered(app)
       app.set :js_compressor, false
 
+      app.after_configuration do
         app.map "/#{app.js_dir}" do
           run JavascriptEnvironment.new(app)
         end
+      end
       # app.map "/#{app.css_dir}" do
       #   run StylesheetEnvironment.new(app)
       # end