Bump gitlab-shell to 8.3.1

8.3.0 was tagged with the commit before the CHANGELOG.md and version.rb bump, so this just fixes a harmless warning.

Bump gitlab-shell to 8.3.1
5a07f03d · Stan Hu · 5d6ada8c · 5a07f03d
Commit 5a07f03d authored 6 years ago by Stan Hu
--- a/GITLAB_SHELL_VERSION
+++ b/GITLAB_SHELL_VERSION
-8.3.0
+8.3.1