Update databases/py-elasticsearch to 1.6.0.

1.6.0 (2015-06-10)
- Add indices.flush_synced API
- helpers.reindex now supports reindexing parent/child documents

1.5.0 (2015-05-18)
- Add support for query_cache parameter when searching
- helpers have been made more secure by changing defaults to raise
  an exception on errors
- removed deprecated options replication and the deprecated benchmark api.
- Added AddonClient class to allow for extending the client from outside

(fhajny)