[gnome-control-center/gnome-3-28] Switch CI to use Fedora 28

[Robert Ancell <rancell src gnome org>]

commit 3e26da421ac2e3bd55cbf18dd2ce918921bcf86e
Author: Robert Ancell <robert ancell canonical com>
Date:   Wed Dec 11 10:58:17 2019 +1300

    Switch CI to use Fedora 28
    
    This was the Fedora version that supported GNOME 3.28. The CI fails to build on
    newer versions of Fedora.

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 711923889..98d4ec7cd 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,4 +1,4 @@
-image: fedora:rawhide
+image: fedora:28
 stages:
   - build
   - test