Bump version to 6.8.1

CHANGELOG

+v 6.8.1
+  - Bump required gitlab-shell version to 1.9.3
+
 v 6.8.0
   - Ability to at mention users that are participating in issue and merge req. discussion
   - Enabled GZip Compression for assets in example Nginx, make sure that Nginx is compiled with --with-http_gzip_static_module flag (this is default in Ubuntu)

VERSION

-6.8.0
+6.8.1