update composer requirements for 2.x elasticsearch

composer.json

@@ -18,7 +18,7 @@
         "phpunit/phpunit": "^4.4|^5.6",
         "squizlabs/php_codesniffer": "~2.0",
         "satooshi/php-coveralls": "~1.0",
-        "elasticsearch/elasticsearch": "~5.0"
+        "elasticsearch/elasticsearch": "~2.0"
     },
     "autoload": {
         "psr-4": {