Fix Webhook payload failed to parse as JSON issue

referencing Issue https://github.com/Strider-CD/strider/issues/428#issuecomment-44067451

req.body.payload is showing up as undefined which is causing payload = JSON.parse(req.body.payload) to fail, the correct code should just be payload = req.body and it works fine, req.body at that point has already done JSON.parse(req.body.payload) at some earlier point?