[gnome-sdk-images] Update webkitgtk4 to 2.18.0

[Adrien Plazas <aplazas src gnome org>]

commit 621e01aa9f7eb669d4eea182d3d9977fc2b6c7f4
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Mon Sep 11 13:53:53 2017 +0200

    Update webkitgtk4 to 2.18.0

 org.gnome.Sdk.json.in |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/org.gnome.Sdk.json.in b/org.gnome.Sdk.json.in
index 052be8d..439b777 100644
--- a/org.gnome.Sdk.json.in
+++ b/org.gnome.Sdk.json.in
@@ -757,8 +757,8 @@
             "sources": [
                 {
                     "type": "archive",
-                    "url": "https://webkitgtk.org/releases/webkitgtk-2.17.4.tar.xz";,
-                    "sha256": "a497eb3c1a8cbc78ccdeb6d7d22557a33ae879e80521d968eaa095a62c9f8526"
+                    "url": "https://webkitgtk.org/releases/webkitgtk-2.18.0.tar.xz";,
+                    "sha256": "b583e46a3de36a3e80ba33b084ead60512a2046aca01ff61e50e519436e5038d"
                 },
                 {
                     "type": "patch",