fix npe in topNBinaryFn with post aggs
Merge pull request #496 from metamx/fix-topN-postagg-npe

fix npe in topNBinaryFn with post aggs
[maven-release-plugin] prepare release druid-0.6.97
[maven-release-plugin] prepare for next development iteration
update versions
[maven-release-plugin] prepare release druid-0.6.98
[maven-release-plugin] prepare for next development iteration