[es6] depend on elasticsearch / elasticsearch_dsl v6

Starting from ES6, `_type` metadata is deprecated,
cf. https://www.elastic.co/guide/en/elasticsearch/reference/current/removal-of-types.html

cubicweb_elasticsearch/__pkginfo__.py

@@ -15,8 +15,8 @@
 
 __depends__ = {'cubicweb': '>= 3.24.0', 'six': '>= 1.4.0',
                'cwtags': None,
-               'elasticsearch': '>=2.0.0,<3.0.0',
-               'elasticsearch-dsl': '<5.0.0',
+               'elasticsearch': '>=6.0.0',
+               'elasticsearch-dsl': '>=6.0.0',
                'beautifulsoup4': None,
                }
 

cubicweb_elasticsearch/ccplugin.py

@@ -115,7 +115,6 @@
                     # "doc_type" in kwargs bellow must be container data.
                     data = {'_op_type': 'index',
                             '_index': index_name or cnx.vreg.config['index-name'],
-                            '_type': json['cw_etype'],
                             '_id': serializer.es_id,
                             '_source': json
                             }