Use https in dub.json

dub.json

@@ -2,7 +2,7 @@
     "name": "gtk-d",
     "targetType": "none",
     "description": "GtkD is the Digital Mars D programing language OO wrapper for Gtk+.",
-    "homepage": "http://gtkd.org",
+    "homepage": "https://gtkd.org",
     "license": "LGPL license with additional exceptions",
     "authors": [
         "Frank Benoit",