Changed DEFAULT_SPARK_EC2_BRANCH from 1.5 to 1.6, added Spark 1.6.1 and 1.6.2 to VALID_SPARK_VERSIONS list, and changed SPARK_EC2_VERSION to latest stable 1.6.2
Changed DEFAULT_SPARK_EC2_BRANCH from 1.5 to 1.6, added Spark 1.6.1
and 1.6.2 to VALID_SPARK_VERSIONS list, and changed SPARK_EC2_VERSION
to latest stable 1.6.2

amplab/spark-ec2,amplab/spark-ec2,madhavhugar/spark-ec2,madhavhugar/spark-ec2,silvaurus/spark-ec2,tomerk/spark-ec2,silvaurus/spark-ec2,madhavhugar/spark-ec2,tomerk/spark-ec2,amplab/spark-ec2,tomerk/spark-ec2,silvaurus/spark-ec2