Fix Build#duration when TimeZone is not UTC

I got wrong Build#durations, cause ServerTime is not UTC.

Sample Output:
```
$: build.from
=> Sat, 30 Mar 2013 16:58:42 UTC +00:00

$: Time.now
 => 2013-03-30 18:20:25 +0100

$: Time.zone.now
 => Sat, 30 Mar 2013 17:20:29 UTC +00:00
```

Using ```Tome.zone.now``` instead of ```Time.now``` fix this issue.