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.

See https://github.com/mher/flower/issues/257#issuecomment-52766817 for details
Merge pull request #261 from jgoclawski/patch-1

Fix recalculating UTC task times in task monitor