Update gitlab-workhorse version to 0.7.8 in installation and update guides

Signed-off-by: Rémy Coutable <remy@rymai.me>

doc/install/installation.md

@@ -398,7 +398,7 @@ If you are not using Linux you may have to run `gmake` instead of
     cd /home/git
     sudo -u git -H git clone https://gitlab.com/gitlab-org/gitlab-workhorse.git
     cd gitlab-workhorse
-    sudo -u git -H git checkout v0.7.7
+    sudo -u git -H git checkout v0.7.8
     sudo -u git -H make
 
 ### Initialize Database and Activate Advanced Features

doc/update/8.9-to-8.10.md

@@ -58,7 +58,7 @@ GitLab 8.1.
 ```bash
 cd /home/git/gitlab-workhorse
 sudo -u git -H git fetch --all
-sudo -u git -H git checkout v0.7.7
+sudo -u git -H git checkout v0.7.8
 sudo -u git -H make
 ```