[gnome-sdk-images] Upgrade to the Gtk 3.24 branch

[Mathieu Bridon <mbridon src gnome org>]

commit ba4d2552a4e59fd8f46965060daad05edc687a84
Author: Mathieu Bridon <bochecha daitauha fr>
Date:   Thu Aug 9 16:04:58 2018 +0200

    Upgrade to the Gtk 3.24 branch

 org.gnome.Sdk.json.in | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/org.gnome.Sdk.json.in b/org.gnome.Sdk.json.in
index 5c2ef0a..000d1d4 100644
--- a/org.gnome.Sdk.json.in
+++ b/org.gnome.Sdk.json.in
@@ -441,7 +441,7 @@
                 {
                     "type": "git",
                     "url": "https://gitlab.gnome.org/GNOME/gtk.git";,
-                    "branch": "gtk-3-22"
+                    "branch": "gtk-3-24"
                 }
             ]
         },