Rails4でGrapeを使ってAPIを作成してHerokuに上げたら本番だけエラーが発生してしまった。

Question

Ruby on Rails4でGrapeを使ってAPIを作ってHeroku上にあげたのですが、本番環境だけ以下のログが出てしまって、Application Errorでサイトが落ちてしまいます。開発環境ではしっかりと動くのですが。。

ログをみると何故か/app/api

2016-04-10T08:45:07.470872+00:00 app[worker.1]: uninitialized constant API::Root::V1
2016-04-10T08:45:07.470893+00:00 app[worker.1]: /app/app/api/api/root.rb:8:in `<class:Root>'
2016-04-10T08:45:07.470894+00:00 app[worker.1]: /app/app/api/api/root.rb:2:in `<module:API>'
2016-04-10T08:45:07.470895+00:00 app[worker.1]: /app/app/api/api/root.rb:1:in `<top (required)>'
2016-04-10T08:45:07.470895+00:00 app[worker.1]: /app/config/routes.rb:55:in `block in <top (required)>'
2016-04-10T08:45:07.470897+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/actionpack-4.2.5/lib/action_dispatch/routing/route_set.rb:434:in `instance_exec'
2016-04-10T08:45:07.470898+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/actionpack-4.2.5/lib/action_dispatch/routing/route_set.rb:412:in `draw'
2016-04-10T08:45:07.470897+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/actionpack-4.2.5/lib/action_dispatch/routing/route_set.rb:434:in `eval_block'
2016-04-10T08:45:07.470899+00:00 app[worker.1]: /app/config/routes.rb:2:in `<top (required)>'
2016-04-10T08:45:07.470900+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/application/routes_reloader.rb:40:in `block in load_paths'
2016-04-10T08:45:07.470900+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/application/routes_reloader.rb:40:in `each'
2016-04-10T08:45:07.470902+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/application/routes_reloader.rb:16:in `reload!'
2016-04-10T08:45:07.470901+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/application/routes_reloader.rb:40:in `load_paths'
2016-04-10T08:45:07.470903+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/application.rb:170:in `reload_routes!'
2016-04-10T08:45:07.470903+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/devise-3.5.6/lib/devise/rails.rb:14:in `block in <class:Engine>'
2016-04-10T08:45:07.470904+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.5/lib/active_support/lazy_load_hooks.rb:36:in `call'
2016-04-10T08:45:07.470905+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.5/lib/active_support/lazy_load_hooks.rb:36:in `execute_hook'
2016-04-10T08:45:07.470906+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.5/lib/active_support/lazy_load_hooks.rb:45:in `block in run_load_hooks'
2016-04-10T08:45:07.470906+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.5/lib/active_support/lazy_load_hooks.rb:44:in `each'
2016-04-10T08:45:07.470909+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/initializable.rb:30:in `instance_exec'
2016-04-10T08:45:07.470910+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/initializable.rb:55:in `block in run_initializers'
2016-04-10T08:45:07.470912+00:00 app[worker.1]: /app/vendor/ruby-2.2.3/lib/ruby/2.2.0/tsort.rb:226:in `block in tsort_each'
2016-04-10T08:45:07.470907+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/activesupport-4.2.5/lib/active_support/lazy_load_hooks.rb:44:in `run_load_hooks'
2016-04-10T08:45:07.470908+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/application/finisher.rb:55:in `block in <module:Finisher>'
2016-04-10T08:45:07.470909+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/initializable.rb:30:in `run'
2016-04-10T08:45:07.470916+00:00 app[worker.1]: /app/vendor/ruby-2.2.3/lib/ruby/2.2.0/tsort.rb:224:in `tsort_each'
2016-04-10T08:45:07.470918+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/initializable.rb:54:in `run_initializers'
2016-04-10T08:45:07.470918+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/railties-4.2.5/lib/rails/application.rb:352:in `initialize!'
2016-04-10T08:45:07.470912+00:00 app[worker.1]: /app/vendor/ruby-2.2.3/lib/ruby/2.2.0/tsort.rb:348:in `block (2 levels) in each_strongly_connected_component'
2016-04-10T08:45:07.470919+00:00 app[worker.1]: /app/config/environment.rb:18:in `<top (required)>'
2016-04-10T08:45:07.470913+00:00 app[worker.1]: /app/vendor/ruby-2.2.3/lib/ruby/2.2.0/tsort.rb:429:in `each_strongly_connected_component_from'
2016-04-10T08:45:07.470920+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/sidekiq-4.1.0/lib/sidekiq/cli.rb:235:in `boot_system'
2016-04-10T08:45:07.470914+00:00 app[worker.1]: /app/vendor/ruby-2.2.3/lib/ruby/2.2.0/tsort.rb:347:in `block in each_strongly_connected_component'
2016-04-10T08:45:07.470921+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/sidekiq-4.1.0/lib/sidekiq/cli.rb:49:in `run'
2016-04-10T08:45:07.470915+00:00 app[worker.1]: /app/vendor/ruby-2.2.3/lib/ruby/2.2.0/tsort.rb:345:in `call'
2016-04-10T08:45:07.470922+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/bin/sidekiq:23:in `load'
2016-04-10T08:45:07.470914+00:00 app[worker.1]: /app/vendor/ruby-2.2.3/lib/ruby/2.2.0/tsort.rb:345:in `each'
2016-04-10T08:45:07.470921+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/gems/sidekiq-4.1.0/bin/sidekiq:12:in `<top (required)>'
2016-04-10T08:45:07.470916+00:00 app[worker.1]: /app/vendor/ruby-2.2.3/lib/ruby/2.2.0/tsort.rb:345:in `each_strongly_connected_component'
2016-04-10T08:45:07.470923+00:00 app[worker.1]: /app/vendor/bundle/ruby/2.2.0/bin/sidekiq:23:in `<main>'
2016-04-10T08:45:07.470917+00:00 app[worker.1]: /app/vendor/ruby-2.2.3/lib/ruby/2.2.0/tsort.rb:203:in `tsort_each'

環境は

• Rails 4.2.5
• ruby 2.2.3p173 (2015-08-18 revision 51636) [x86_64-darwin14]

routes.rb

  mount API::Root => "/"

application.rb

config.paths.add File.join('app', 'api'), glob: File.join('**', '*.rb')
config.autoload_paths += Dir[Rails.root.join('app', 'api', '*')]

config.middleware.use(Rack::Config) do |env|
  env['api.tilt.root'] = Rails.root.join 'app', 'views', 'api'
end

フォルダの階層

/app

/api

/api

root.rb
/V1

root.rb
articles.rb

/app/api/api/root.rb

module API
  class Root < Grape::API
    prefix "api"
    format :json
    formatter :json, Grape::Formatter::Jbuilder
    mount V1::Root
  end
end

/app/api/api/V1/root.rb

module API
  module V1
    class Root < Grape::API
      version 'v1', :using => :path
      mount V1::Articles
    end
  end
end

/app/api/api/V1/articles.rb

module API
  module V1
    class Articles < Grape::API
      format :json
      formatter :json, Grape::Formatter::Jbuilder
      resource :articles do
        get "/" do
          @articles = @articles.all
        end
      end
    end
  end
end

以上のようになっていて、localhostではちゃんとJSONが出力されるのですが、本番環境(Heroku)にプッシュすると上記のようなエラーが発生してしまって、その原因がわかりません。
エラーによると、/app/app/api/api/...と階層の最初に/appが何故か追加されていて、おそらくこのせいでRailsの命名規則？みたいなのに従ってないからダメ！みたいなことだと思うんですが、何故Herokuにあげたらこの/appという階層が追加されたかも理由がさっぱりです。
分かるかたいたら宜しくお願いします！！