Change to Clock[F].instantNow in 2.3.1

[jbwheatley]

change due to #1461

More of a PSA for anyone else who suddenly got tests failing when upgrading to CE 2.3.1. If you had something like this in your test suites:

private val mockClock: Clock[IO] = new Clock[IO] {
      def realTime(unit: TimeUnit): IO[Long]  = IO(123456L)
      def monotonic(unit: TimeUnit): IO[Long] = IO(123456L)
}

and anything you are testing used Clock[F].instantNow to give the current time as an instant, it now gives you EPOCH + 123456 nanos, not EPOCH + 123456millis