GitLab app is not threadsafe so better disable allow_concurrency for production env

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

config/environments/production.rb

@@ -83,4 +83,6 @@ Gitlab::Application.configure do
   config.eager_load = true
   config.assets.js_compressor = :uglifier
+
+  config.allow_concurrency = false
 end