[gnome-sdk-images] Update to WebKitGTK+ 2.21.91

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 8b39aaeed15b77f0c6b490ae27017e871a1fcda8
Author: Michael Catanzaro <mcatanzaro igalia com>
Date:   Thu Aug 16 11:07:52 2018 -0500

    Update to WebKitGTK+ 2.21.91

 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 f770204..42c8973 100644
--- a/org.gnome.Sdk.json.in
+++ b/org.gnome.Sdk.json.in
@@ -885,8 +885,8 @@
             "sources": [
                 {
                     "type": "archive",
-                    "url": "https://webkitgtk.org/releases/webkitgtk-2.21.5.tar.xz";,
-                    "sha256": "c31ec8c701dc174a6898fc78f08b549ec986abb5e6c7e86db46680a9ee45821f"
+                    "url": "https://webkitgtk.org/releases/webkitgtk-2.21.91.tar.xz";,
+                    "sha256": "d9a35a84232c5d6ad7d0bb704918e110d30f8592f233770c830431a384c3bb0c"
                 }
             ]
         },