Update docs to use 6-1-stable branch

doc/install/installation.md

@@ -153,10 +153,10 @@ To setup the MySQL/PostgreSQL database and dependencies please see [`doc/install
     cd /home/git/gitlab
     # Checkout to stable release
-    sudo -u git -H git checkout 6-0-stable
+    sudo -u git -H git checkout 6-1-stable
 **Note:**
-You can change `6-0-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
+You can change `6-1-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
 ## Configure it