rubygems在ubuntu下的升级

在ubuntu下升级rubygems报以下错误
（*ruby版本1.9.2
   rubygems版本1.3.7
）

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.

解决方案

Update to 1.8.11: sudo gem install rubygems-update -v 1.8.11
sudo update_rubygems
sudo gem update --system