Fix recalculating UTC task times in task monitor

Fixes #257

I believe that `task.timestamp` should not be touched, as everything is in UTC now.

Need someone who knows the project better, to verify if my assumptions are correct. I know the code fixes the issue, but maybe introduces other bugs, which I'm not aware of.

See https://github.com/mher/flower/issues/257#issuecomment-52766817 for details