Hello,

This upgrades racket to 6.5 which should fix build failure on hydra
(currently unavailable). The analogous bug report on nix is
here: https://github.com/NixOS/nixpkgs/issues/13733.

The patch mentions it is fixed after the 6.4 release,
https://github.com/NixOS/nixpkgs/commit/927ce5ca718d0bbba69a062534b296a42595ef50,
so it should definitely be fixed on 6.5.

Cheers,
Alex