changelog.rst: Replace 1.6.7 with 1.6.8.
changelog.rst: Replace 1.6.7 with 1.6.8.

FeodorFitsner/mpc-hc.org,kasper93/mpc-hc.org,Underground78/mpc-hc.org,Underground78/mpc-hc.org,mpc-hc/mpc-hc.org,mpc-hc/mpc-hc.org,FeodorFitsner/mpc-hc.org,Underground78/mpc-hc.org,kasper93/mpc-hc.org,FeodorFitsner/mpc-hc.org,mpc-hc/mpc-hc.org