Update ElasticSearch and Elastica to 6.x

[anvit]

Update ElasticSearch and Elastica and address issues introduced by breaking changes in ElasticSearch 6.x.

The most notable changes are:

• Dropped support for multiple mapping types: ES 6.x no longer allows using multiple indices, They suggest handling this by creating multiple indices, each with its own type.
• Mapping changes: The include_in_all mapping parameter is disallowed and the _all field is disabled by default.

Changes in Elastica:

• Elastica only has a addDocuments, deleteDocuments, and updateDocuments methods in 6.x and there are no longer addDocument, deleteDocument, updateDocument methods for handling a singular documents.