installation documentation: updated to use latest stable 7.7 branch

doc/install/installation.md

@@ -181,9 +181,9 @@ 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 7-6-stable gitlab
+    sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-ce.git -b 7-7-stable gitlab
 
-**Note:** You can change `7-6-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
+**Note:** You can change `7-7-stable` to `master` if you want the *bleeding edge* version, but never install master on a production server!
 
 ### Configure It