Problem: travis darwin rebuilds Nix
Problem: travis darwin rebuilds Nix

Solution: Bump to latest racket2nix, which has fractalide/racket2nix#117 .

This will halve the time spent, from 1600 s to 800 s, when testing on darwin.

fractalide/racket2nix#117 bumps its nixpkgs to same rev as ours, so this also
saves us one download of nixpkgs.

sjmackenzie/fractalide,sjmackenzie/fractalide,fractalide/fractalide,sjmackenzie/fractalide,fractalide/fractalide,dmichiels/fractalide,dmichiels/fractalide,fractalide/fractalide,dmichiels/fractalide