Fix UTC JSON instance.

Doing some round-trip testing through CouchDB using the Arbitrary
instances, I was ending up with strings with a superfluous '.' before
the time zone 'Z', which was causing them to fail to be read back in.

These tests have been rock-solid using ZonedTime, but I recently made
the decision to start storing UTCTime, and suddenly failures started
springing up---so I know my round-trip strategy should work.

I removed the storing of millis entirely, and things went back to
working.