Fixed #139 - Django-celery 2.5.5: celerycam shows a stacktrace

See also https://groups.google.com/forum/#!msg/celery-users/Eb-DpLw77R8/rN264yWmXswJ

The problem is caused by MySQL's inability to store tz-aware timestamps.

This change makes `TaskState` convert the `eta` attribute to UTC before saving.