Make script get farther on Mac OS X 10.6.8 and VirtualBox 4.2.16

I had weird issues with the script failing to even upgrade Nix, which I solved by using 1.5.3 as the base vs. 1.5.2.

Then I ran into trouble installing nixops, which I solved by replacing the "-f" argument with "-I".

Unfortunately, it still doesn't quite work for me: the VM comes up, but the process gets stuck on "waiting for IP..." even though I've configured the host-only adapter and DHCP. Pretty extensive searching and experimentation suggests making host-only networking in VirtualBox work is highly problematic, so any further advice here would be most welcome.

Thanks for creating this!