[gnome-builder] build: switch to 3.28 for CI

[Christian Hergert <chergert src gnome org>]

commit 570886bd927264ea70d44634f6cdc0d2a9c829dd
Author: Christian Hergert <chergert redhat com>
Date:   Tue May 22 11:49:08 2018 -0700

    build: switch to 3.28 for CI

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index f71f206f2..5066417fe 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -2,7 +2,7 @@ stages:
     - test
 
 flatpak:
-    image: registry.gitlab.com/alatiera/gnome-nightly-oci/gnome-master:latest
+    image: registry.gitlab.gnome.org/gnome/gnome-runtime-images/gnome:3.28
     stage: test
 
     variables: