Install latest version of GitLab Shell, commit in master did not merge cleanly.

doc/install/installation.md

@@ -122,7 +122,7 @@ GitLab Shell is an ssh access and repository management software developed speci
     cd gitlab-shell
     # switch to right version
-    sudo -u git -H git checkout v1.7.4
+    sudo -u git -H git checkout v1.7.9
     sudo -u git -H cp config.yml.example config.yml