`nix-env -u` didn't work -> had to clone nixpkgs see instructions for OS X: https://nixos.org/wiki/Nix_on_OS_X#Using_Nix_on_10.9_and_10.10
`nix-env -u` didn't work -> had to clone nixpkgs
see instructions for OS X: https://nixos.org/wiki/Nix_on_OS_X#Using_Nix_on_10.9_and_10.10

vogler/prezto