Update gitlab-shell version in installation and update documentation

installation was totally off compared to the update document. The update
document was slightly off (rake gitlab:check excpects 2.5.4 and not
2.5.3)

doc/install/installation.md

@@ -280,7 +280,7 @@ 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[v2.4.3] REDIS_URL=unix:/var/run/redis/redis.sock RAILS_ENV=production
+    sudo -u git -H bundle exec rake gitlab:shell:install[v2.5.4] 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:

doc/update/7.7-to-7.8.md

@@ -37,7 +37,7 @@ sudo -u git -H git checkout 7-8-stable-ee
 ```bash
 cd /home/git/gitlab-shell
 sudo -u git -H git fetch
-sudo -u git -H git checkout v2.5.3
+sudo -u git -H git checkout v2.5.4
 ```
 
 ### 4. Install libs, migrations, etc.