Update stable branch in installation documentation. Fixes #2191

doc/install/installation.md

@@ -195,9 +195,9 @@ We recommend using a PostgreSQL database. For MySQL check [MySQL setup guide](da
 ### Clone the Source
 
     # Clone GitLab repository
-    sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-ce.git -b 7-12-stable gitlab
+    sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-ce.git -b 7-13-stable gitlab
 
-**Note:** You can change `7-12-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
+**Note:** You can change `7-13-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
 
 ### Configure It