Upgrade to moment-timezone-0.0.6

- Remove workaround in fieldHelpers.js to allow parsing dates assuming
  a certain timezone as moment.tz() supports that now.
- Adjust assumptions about moment-timezone which seems to handle
  crossing timezone changing points better now.
Merge pull request #712 from marcphilipp/moment-timezone-0.0.6

Upgrade to moment-timezone-0.0.6