Log Message:
Update databases/py-elasticsearch to 6.0.0.

6.0.0 (2017-11-14)
- compatibility with Elasticsearch 6.0.0

5.5.0 (2017-11-10)
- streaming_bulk helper now supports retries with incremental backoff
- scan helper properly checks for successful shards instead of just
  checking failed
- compatible release with elasticsearch 5.6.4
- fix handling of UTF-8 surrogates