[root@tdn-VirtualBox]~ #gem update --system      
ERROR:  While executing gem ... (RuntimeError)
    gem update --system is disabled on Debian, because it will overwrite the content of the rubygems Debian package, and might break your Debian system in subtle ways. The Debian-supported way to update rubygems is through apt-get, using Debian official repositories.
If you really know what you are doing, you can still update rubygems by setting the REALLY_GEM_UPDATE_SYSTEM environment variable, but please remember that this is completely unsupported by Debian.
[root@tdn-VirtualBox]~ #