At the time of writing this book, the main distribution was Debian Wheezy; but Debian Jessie was added around June 2013 for testing. A lot of game server packages were updated only in Jessie because of newer dependencies.
You can upgrade the entire 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.
We should add the following two lines into /etc/apt/sources.list:
deb http://mirrordirector.raspbian.org/raspbian/ jessie main contrib non-free rpi deb http://archive.raspbian.org/raspbian jessie main contrib non-free rpi
The next step is to tell aptitude to use the wheezy repository for normal updates and that you would like to use Jessie from time to time. These settings should be typed into the file found at /etc/apt/preferences. Create this...