[gnome-sdk-images] Bump vala to 0.40.

[Michael Gratton <mjog src gnome org>]

commit c000b4bbd9b7fbac000609a59329a85aae9c6ade
Author: Michael James Gratton <mike vee net>
Date:   Sat May 19 10:02:05 2018 +1000

    Bump vala to 0.40.

 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 0b6a17a..f579ece 100644
--- a/org.gnome.Sdk.json.in
+++ b/org.gnome.Sdk.json.in
@@ -190,7 +190,7 @@
                 {
                     "type": "git",
                     "url": "https://git.gnome.org/browse/vala";,
-                    "branch": "0.36"
+                    "branch": "0.40"
                 }
             ]
         },