Intermittent HTTP 403 on callbacks from Retriever

Retriever is intermittently getting a 403 forbidden response from Shepherd when POSTing a callback. It appears this is actually an nginx config issue, rather than a strictly Shepherd issue; this is the response body:

```html
<html>
<head><title>403 Forbidden</title></head>
<body>
<center><h1>403 Forbidden</h1></center>
<hr><center>nginx</center>
</body>
</html>
```