gem update --system on ubuntu fails

Doing a "gem update --system" on Ubuntu gave the following error:

ERROR: While executing gem ... (RuntimeError)
gem update --system is disabled on Debian. RubyGems can be updated using the official Debian repositories by aptitude or apt-get.


A manual working solution:

gem install rubygems-update
cd /var/lib/gems/1.8/bin
./update_rubygems

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