update version in installation doc

doc/install/installation.md

@@ -173,13 +173,13 @@ 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 6-7-stable gitlab
+    sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-ce.git -b 6-8-stable gitlab
     # Go to gitlab dir
     cd /home/git/gitlab
 **Note:**
-You can change `6-7-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
+You can change `6-8-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
 ## Configure it