setup sprockets after config parse

lib/middleman/core_extensions/sprockets.rb

@@ -5,8 +5,10 @@ module Middleman::CoreExtensions::Sprockets
     def registered(app)
       app.set :js_compressor, false
 
-      app.map "/#{app.js_dir}" do
-        run JavascriptEnvironment.new(app)
+      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)