Previously: 5.1 > 5.2 > luajit
Now: 5.2 > luajit > 5.1
I randomly decided that this should be done, since I suspect most
environments will prefer the highest Lua version anyway. There is not
much of a point picking the older one by default.
Maybe 5.1 should be dropped fully, but considering we need to stay
compatible with luajit, there is no particular incentive for this.