racket 6.8

The attached patch advances the version of the racket expression in
nixpkgs to the latest released 6.8 version.

From 815aae487d5ed4b70145ebadc03a5bd040a8a829 Mon Sep 17 00:00:00 2001
From: Karn Kallio <kkallio@skami.org>
Date: Fri, 27 Jan 2017 22:55:18 -0400
Subject: [PATCH] racket: advance to the 6.8 latest released version.