Updating to Jessie
At the time of writing this book, the main distribution for the Rasbperry Pi was still Debian Wheezy. Debian Jessie was added around June 2013 for testing on the Rasspbery Pi but is already used on desktop machines. As of April 2015, there seems to be a stable version for the Raspberry Pi. A lot of game server packages were updated only in Jessie because of newer dependencies.
You can upgrade the entire Raspberry Pi to Jessie, but you are advised not to do this until the official image is released. Instead, we will install selected, specific packages from Jessie while keeping the entire distribution stable in Wheezy. These steps can be skipped if you are already using Jessie.