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

This will only affect restify users using bunyan 0.18.3 or 0.19.0 that *also set custom serializers*. The serializers in restify itself have sufficient guards to not be hit by this. If the user is using restify's bunyan node_module, then restify versions 2.3.2, 2.3.3 and 2.3.4 are affected.