version of bundler installed by helper is not controled

The current version of module install latest and that all.

https://gitlab.adullact.net/adullact/puppet-gitlabrunner/blob/6145437432e5281a30fd2980f6d063328966f2ce/manifests/helper/shell.pp#L151

It should be useful to be able to control the version installed for bundler gem.