[gnome-sdk-images] Update to WebKitGTK+ 2.21.4

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 7d6c523ce080d845a6251535f83d6714472b6a65
Author: Michael Catanzaro <mcatanzaro igalia com>
Date:   Wed Jun 13 16:32:25 2018 -0500

    Update to WebKitGTK+ 2.21.4

 org.gnome.Sdk.json.in | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/org.gnome.Sdk.json.in b/org.gnome.Sdk.json.in
index 675563a..66ed30d 100644
--- a/org.gnome.Sdk.json.in
+++ b/org.gnome.Sdk.json.in
@@ -878,8 +878,8 @@
             "sources": [
                 {
                     "type": "archive",
-                    "url": "https://webkitgtk.org/releases/webkitgtk-2.21.3.tar.xz";,
-                    "sha256": "d30dc877f71af260f9f7ce0a75544ee9cc38747b3889023bf7ff4c83e8b2b8dd"
+                    "url": "https://webkitgtk.org/releases/webkitgtk-2.21.4.tar.xz";,
+                    "sha256": "59f4359ace702ecaa92b975666c1bcde3858258909b047e7675ce3807efdd1e7"
                 }
             ]
         },