The Robbit Launcher has been updated to version 3.0.7u4. This version adds the ability to choose how much RAM you want to allocate to the game, which can be accessed from the 'settings' screen after adding/installing the game. The default continues to be 2GB on 64-bit systems but you can increase it now in 1GB increments up to a max of 8GB.
The newest builds seem to need more than the originally allocated 2GB in some instances, especially during world generation which with only 2GB allocated seems to result in some 'GC limit reached' crashes. I figured I'd put in this patch so if you are having RAM exhaustion issues in single player you can increase the RAM allocation at your own leisure.
(Please bear in mind that larger RAM allocation can sometimes be detrimental to performance because they will lead to significantly longer GC stalls, so change this setting with caution.)