fpm binary not found on Debianoids if installed via gem_package

System ruby packages on Debian based systems put ruby and gem binaries in `/usr/bin/` and thus Ohai will set `node['languages']['ruby']['bin_dir']` to it. But system rubygem installs its binaries by default to either `$(gem env gemdir)/bin/` or `/usr/local/bin/` (depending on the distro and version).
So `node['fpm_tng']['exec']` pointing to `/usr/bin/fpm` is most probably wrong in these cases.

I'm not even sure what would be the best fox for this. Obviously the ruby is not necessarily even installed from the distro package.

Any thoughts?