This one brings official linux support to the game for the first time, as well as a 64bit build for OSX. There are also a couple of other minor tweaks in here for your enjoyment.
This is a standard update that you can download through the in-game updater itself, if you already have any version of the game. When you launch the game, hit the big “Play” button and then you’ll see the notice of the update having been found if you’re connected to the Internet at the time. As of version 1.500, you’ll also see the notice of updates having been found in the upper left corner of the main menu itself.