switch nix-mode out of melpaStable to melpa
switch nix-mode out of melpaStable to melpa

the version of nix-mode in melpaStable still had this issue:
https://github.com/NixOS/nix-mode/issues/125

mjhoy/dotfiles,mjhoy/dotfiles,mjhoy/dotfiles,mjhoy/dotfiles,mjhoy/dotfiles