added missing elasticsearch from the required dev

composer.json

@@ -16,6 +16,7 @@
         "paragonie/random_compat": "^1.0|^2.0"
     },
     "require-dev": {
+        "elasticsearch/elasticsearch": "^6.0",
         "phpunit/phpunit": "^6.0",
         "squizlabs/php_codesniffer": "^3.0",
         "satooshi/php-coveralls": "^2.0"