Made `freshTimestamp` return a Carbon object instead of DateTime

as Carbon is already used in database I think this is useful for unit testing

I can do `Carbon::setTestNow(Carbon::createFromDate(2000, 1, 1));` to fake the "created_at" field in an easier way.

I hope this will be merged in
Merge pull request #2262 from onigoetz/patch-4

Made `freshTimestamp` return a Carbon object instead of DateTime