Use trigger.time (if available) for rate-limiting

[dgw]

Added in sopel 6.2.1, so obviously depends on #8 (which is coming, now that my bot's running 6.x).

Using trigger.time means rate-limiting will use the value of server-time if available. No real change, but should make the rate limit more accurate (not that anyone ever complains about it being off by a second or two).