Last-Modified header is incorrect when server time zone is not GMT

Hi!

I came across a small bug with the Last-Modified header that is generated by Slim. It uses PHP's `DATE_RFC1123`, but this is not sufficient to meet the standards of the HTTP specification. Specifically, the [specification](http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.3.1) requires that:

    All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception.

However the output of `date( DATE_RFC1123, $time )` depends on the server time zone:

```php
$time = time();
date_default_timezone_set("GMT");
echo date( DATE_RFC1123, $time );
date_default_timezone_set("Africa/Nairobi");   // GMT+3
echo date( DATE_RFC1123, $time );
```
The output is:
```
Mon, 15 Jul 2013 12:27:30 GMT   // Correctly formatted
Mon, 15 Jul 2013 15:27:30 +0300    // Not correct - HTTP timestamps must be in GMT
```

Patch fixes it to ensure that the timestamp always adheres to the format required by the HTTP specification.