Use gitlab-workhorse 0.7.11

Versions 0.7.9 and 0.7.10 crash on previously valid 'authBackend'
settings.

GITLAB_WORKHORSE_VERSION

-0.7.10
+0.7.11

doc/install/installation.md

@@ -397,7 +397,7 @@ 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 v0.7.10
+    sudo -u git -H git checkout v0.7.11
     sudo -u git -H make
 ### Initialize Database and Activate Advanced Features