Increase proxy_buffers to avoid upstream header issue in Firefox

This is to address the following issue, seen only in Firefox (?):

```
2014/10/30 02:25:26 [error] 9#0: *69 upstream sent too big header while reading response header from upstream, client: [redacted], server: , request: "GET / HTTP/1.1", upstream: "[redacted]", host: "[redacted]"
```