[WIP] Fix purge syntax for Varnish 4.0.1

This fixes #97.
Without the `usleep()` response looks like this:

```http
HTTP/1.1 200 OK
Host: localhost:6181
X-Powered-By: PHP/5.5.14-1~dotdeb.1
Cache-Control: max-age=3600
Content-Type: text/html
X-Cache-Debug: 1
Vary: Accept
Date: Fri, 18 Jul 2014 11:18:24 GMT
x-url: /negotation.php
x-host: localhost:6181
X-Varnish: 32773 32774
Age: 0
Via: 1.1 varnish-v4
x-cache: HIT
Content-Length: 0
Connection: keep-alive
Accept-Ranges: bytes
```

And becomes this with `usleep()`:

```http
HTTP/1.1 200 OK
Host: localhost:6181
X-Powered-By: PHP/5.5.14-1~dotdeb.1
Cache-Control: max-age=3600
Content-Type: text/html
X-Cache-Debug: 1
Vary: Accept
Date: Fri, 18 Jul 2014 11:18:55 GMT
x-url: /negotation.php
x-host: localhost:6181
X-Varnish: 32773
Age: 0
Via: 1.1 varnish-v4
x-cache: MISS
Content-Length: 0
Connection: keep-alive
Accept-Ranges: bytes
```

As you can see, the content type is `text/html` in both cases, i.e. different from the `application/json` above, so it seems no stale content is served. Adding a `microtime()` in `negotation.php` verifies that the response served is different from the `application/json` response in line 104 above.