[gnome-initial-setup] ci: use Fedora 31

[Will Thompson <wjt src gnome org>]

commit 1d381c5497ee90208c867cca00ac86f379840b6c
Author: Will Thompson <will willthompson co uk>
Date:   Sat Jan 4 20:11:41 2020 +0000

    ci: use Fedora 31
    
    This will allow us to depend on systemd ≥ 242, in particular (!66).

 .gitlab-ci.yml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 38ce3e9..098da0a 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,4 +1,4 @@
-image: fedora:30
+image: fedora:31
 
 stages:
   - build