switch gitlab-shell to correct version

a gitlab:check does say that you need v1.1.0 of gitlab-shell...

doc/install/installation.md

@@ -106,6 +106,11 @@ GitLab Shell is a ssh access and repository management software developed specia
     git clone https://github.com/gitlabhq/gitlab-shell.git
     cd gitlab-shell
+    
+    # switch to right version for v5.0
+    git checkout v1.1.0
+    git checkout -b v1.1.0
+    
     cp config.yml.example config.yml
     # Edit config and replace gitlab_url