Update version to 1.6.0-SNAPSHOT.

R/pkg/DESCRIPTION

assembly/pom.xml

bagel/pom.xml

core/pom.xml

core/src/main/scala/org/apache/spark/package.scala

docs/_config.yml

examples/pom.xml

external/flume-assembly/pom.xml

external/flume-sink/pom.xml

external/flume/pom.xml

external/kafka-assembly/pom.xml

external/kafka/pom.xml

external/mqtt-assembly/pom.xml

external/mqtt/pom.xml

external/twitter/pom.xml

external/zeromq/pom.xml

extras/java8-tests/pom.xml

extras/kinesis-asl-assembly/pom.xml

extras/kinesis-asl/pom.xml

extras/spark-ganglia-lgpl/pom.xml