Debian is in my personal opinion the best Linux server distribution ever! I’ve never used something else. But one big drawback is that a lot of stuff is really outdated: With PHP you have to use backports. Java and Maven brings some hassle, too. And now I realized the same problem with Ruby Gems. But there is a solution as described here (german blog post).
By default Debian disables the gem update –system to prevent circumventing the Debian packet management system. If you need a newer Gem version you can install the rubygems-update:
$> sudo gem install rubygems-update --version=1.8.20
With the –version option you can chose your preferred version. After installation you need to invoke the update command:
$> sudo update_rubygems