Explicitly set elasticsearch/elasticsearch dependency

elasticsearch/elasticsearch 6.8.0 was released missing
some class aliases, so currently breaks expected semver
compatibility.

Therefore explicitly require elasticsearch/elasticsearch
~6.5.1 and ~6.7.2.

6.5.x supports ES >= 6.0, < 6.6; 6.5.4 is currently used
in WMF prod.
6.7.x supports ES >= 6.6, < 7.0.

This should be reverted when upstream bugs are fixed.

Upstream tasks:
* ruflin/Elastica#1908
* elastic/elasticsearch-php#1112

Bug: T276320
Bug: T276854
Change-Id: If00e4f871dbfa5572563f1eb5f60ded51f79a06b

composer.json

@@ -13,6 +13,7 @@
 	],
 	"require": {
 		"ruflin/elastica": "6.1.3",
+		"elasticsearch/elasticsearch": "~6.5.1||~6.7.2",
 		"ext-curl": "*"
 	},
 	"require-dev": {