OAK-1169 - Update Guava to version 15.0

Preparing for the update to 15.0

Replacing the usage of deprecated com.google.common.io.NullOutputStream with
ByteStreams.nullOutputStream() as this class has been removed in 15.0

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1541011 13f79535-47bb-0310-9956-ffa450edef68