Bump gitlab-shell to v2.6.6

GITLAB_SHELL_VERSION

-2.6.5
+2.6.6

doc/install/installation.md

@@ -310,7 +310,7 @@ We recommend using a PostgreSQL database. For MySQL check [MySQL setup guide](da
 GitLab Shell is an SSH access and repository management software developed specially for GitLab.
 
     # Run the installation task for gitlab-shell (replace `REDIS_URL` if needed):
-    sudo -u git -H bundle exec rake gitlab:shell:install[v2.6.5] REDIS_URL=unix:/var/run/redis/redis.sock RAILS_ENV=production
+    sudo -u git -H bundle exec rake gitlab:shell:install[v2.6.6] REDIS_URL=unix:/var/run/redis/redis.sock RAILS_ENV=production
 
     # By default, the gitlab-shell config is generated from your main GitLab config.
     # You can review (and modify) the gitlab-shell config as follows: