Round the time values to the Rethinkdb precision (#5774)
Round the time values to the Rethinkdb precision (#5774)

Ruby seems to add some garbage beyond the provided epoch_time resolution, which makes the read times be slightly after the originally saved times. This is specially evident when running tests.

```
2.2.4 :006 > Time.at(1463567776.592).iso8601(30)
 => "2016-05-18T12:36:16.592000007629394531250000000000+02:00"
2.2.4 :007 > Time.at(1463567776.592).round(3).iso8601(30)
 => "2016-05-18T12:36:16.592000000000000000000000000000+02:00"
```
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