use NODE_ENV=production when compiling assets in upgrader script

lib/gitlab/upgrader.rb

@@ -67,7 +67,10 @@ module Gitlab
     end
 
     def env
-      { 'RAILS_ENV' => 'production' }
+      {
+        'RAILS_ENV' => 'production',
+        'NODE_ENV' => 'production'
+      }
     end
 
     def upgrade