I'm using the Apple slim aluminum keyboard, which doesn't have the SysRq key:
Just did this. If you don't want to recompile the kernel like suggested in the first link of the first answer, there is a simpler way. It's kind of outlined in the second link of the first answer, but here is the actual procedure:
Now it should be possible to press Alt+F13 and issue REISUB. Worked on my Xubuntu 15.10.