[gnome-builder] build: switch to master SDK

[Christian Hergert <chergert src gnome org>]

commit cc3b47406f2248fbc6ede65e70fe9e23217756d6
Author: Christian Hergert <chergert redhat com>
Date:   Tue Aug 7 15:32:55 2018 -0700

    build: switch to master SDK

 build-aux/flatpak/org.gnome.Builder.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/build-aux/flatpak/org.gnome.Builder.json b/build-aux/flatpak/org.gnome.Builder.json
index 5cbd3e4cd..050b0b720 100644
--- a/build-aux/flatpak/org.gnome.Builder.json
+++ b/build-aux/flatpak/org.gnome.Builder.json
@@ -1,7 +1,7 @@
 {
     "app-id" : "org.gnome.Builder",
     "runtime" : "org.gnome.Sdk",
-    "runtime-version" : "3.28",
+    "runtime-version" : "master",
     "sdk" : "org.gnome.Sdk",
     "command" : "gnome-builder",
     "tags" : [