undefined method `path_separator' for Gem:Module

The version of Gem being executed is too old to have that method.

You can update with:

gem update --system

But there may be other issues with running your app on an old version of Ruby like 1.9. Currently, only versions 2.2 and up are supported.

You may want to look into telling Jenkins to use the same version of Ruby that you use in your runtime environment with RVM or Docker.

CodeHunter