Know what you are doing and want a stable version? Just use the version from nixpkgs.
nix profile install nixpkgs#crate2nixcrate2nix help
nix profile install github:nix-community/crate2nixcrate2nix help
nix-channel --update # if you wishnix-env -i -f '<nixpkgs>' -A crate2nix
nix-env -i -f https://github.com/nix-community/crate2nix/tarball/master