[gtk] flatpak: Build against the master runtime

[Matthias Clasen <matthiasc src gnome org>]

commit 1a671cfd635c85500e9e75760a923a67ba0e45a0
Author: Matthias Clasen <mclasen redhat com>
Date:   Fri Feb 7 09:15:46 2020 -0500

    flatpak: Build against the master runtime
    
    This is what the ci is set up to use, and building
    against 3.34 fails there.

 build-aux/flatpak/org.gtk.Demo4.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/build-aux/flatpak/org.gtk.Demo4.json b/build-aux/flatpak/org.gtk.Demo4.json
index 4300c45e1e..acc57b3750 100644
--- a/build-aux/flatpak/org.gtk.Demo4.json
+++ b/build-aux/flatpak/org.gtk.Demo4.json
@@ -1,7 +1,7 @@
 {
     "app-id": "org.gtk.Demo4",
     "runtime": "org.gnome.Platform",
-    "runtime-version": "3.34",
+    "runtime-version": "master",
     "sdk": "org.gnome.Sdk",
     "command": "gtk4-demo",
     "tags": ["devel", "development", "nightly"],