Fixed critical alerts sending to admin users

refs https://github.com/TryGhost/Team/issues/726

- The refed feature got broken during the refactors. Even though this area is covered by unit tests the "this context" testing should probably done on an integration test level, which we don't have a clear pattern for just yet

core/server/update-check.js

@@ -53,7 +53,7 @@ module.exports = async () => {
             ghostVersion: ghostVersion.original
         },
         request,
-        sendEmail: ghostMailer.send
+        sendEmail: ghostMailer.send.bind(ghostMailer)
     });
 
     await updateChecker.check();