SPARK-1767: Prefer HDFS-cached replicas when scheduling data-local tasks

This change reorders the replicas returned by
HadoopRDD#getPreferredLocations so that replicas cached by HDFS are at
the start of the list.  This requires Hadoop 2.5 or higher; previous
versions of Hadoop do not expose the information needed to determine
whether a replica is cached.
locs.getOrElse(split.getLocations.filter(_ != "localhost")) is more straightforward to my eyes.  And if this is a hot codepath the slightly-slower match syntax might be a concern