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
Merge pull request #141 from Deradon/patch-1

Fix Build#duration when TimeZone is not UTC