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.

Author: Colin Patrick Mccabe <cmccabe@cloudera.com>

Closes #1486 from cmccabe/SPARK-1767 and squashes the following commits:

338d4f8 [Colin Patrick Mccabe] SPARK-1767: Prefer HDFS-cached replicas when scheduling data-local tasks