update to bunyan 0.20.0, bunyan 0.18.3 introduce a serializer bug

Details:
https://github.com/trentm/node-bunyan/blob/master/CHANGES.md#bunyan-0183
Merge pull request #348 from trentm/better-bunyan

update to bunyan 0.20.0, bunyan 0.18.3 introduce a serializer bug