Add utc method to timestamp

[niallzato]

Follow up on this PR. https://github.com/segmentio/analytics_flutter/pull/105/files

We also need to set the timestamp to UTC as per our docs: https://segment.com/docs/connections/spec/common/#timestamps

Every API call has four timestamps, originalTimestamp, timestamp, sentAt, and receivedAt. They’re used for very different purposes.

All timestamps are ISO-8601 date strings, and are in the UTC timezone. To see the user’s timezone information, check the timezone field that’s automatically collected by client-side libraries.