update nginx.conf

Without the ``return 200`` nginx throws errors like:

```
2014/09/06 00:02:24 [error] 11#0: *1 open() "/usr/share/nginx/html/v1/_ping" failed (2: No such file or directory), client: ip, server: example.com, request: "GET /v1/_ping HTTP/1.1", host: "example.com"
```