[gtranslator] Fix CI with gnome runtime 3.28

[Daniel Garcia Moreno <danigm src gnome org>]

commit cd72767aa3ee868c5de5047bca7f171180f20c77
Author: Daniel García Moreno <danigm wadobo com>
Date:   Tue Sep 18 12:55:11 2018 +0200

    Fix CI with gnome runtime 3.28

 .gitlab-ci.yml | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index d3d8d82e..60ca6c81 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -16,7 +16,7 @@ variables:
       - .flatpak-builder/cache/
 
 flatpak:
-  image: "registry.gitlab.gnome.org/gnome/gnome-runtime-images/gnome:master"
+  image: "registry.gitlab.gnome.org/gnome/gnome-runtime-images/gnome:3.28"
   stage: "test"
   script:
     - flatpak-builder --stop-at=${FLATPAK_MODULE} --force-clean app ${MANIFEST_PATH}
@@ -64,4 +64,4 @@ stop_review:
     action: stop
   except:
     - master@GNOME/gtranslator
-    - tags
+    - tags
\ No newline at end of file