Update to current nix / nixops

This updates the scripts to be useable again, with the current versions of nix (1.8pre) / nixops (1.3pre), including a workaround for an outdated nix inside the VMs which prevents deployment.

Tested on 10.9 with XCode 5.1 and 10.10. with XCode 6.1

It fetches the latest binary builds for nix and nixops - this should be pinned to the release versions once available.

This also fixes #2.