SPARK-1829 Sub-second durations shouldn't round to "0 s"

As "99 ms" up to 99 ms
As "0.1 s" from 0.1 s up to 0.9 s

https://issues.apache.org/jira/browse/SPARK-1829

Compare the first image to the second here: http://imgur.com/RaLEsSZ,7VTlgfo#0