Remove Amazon Elasticsearch Service version warning

Amazon Elasticsearch Service now supports Elasticsearch 2.3.
See: https://aws.amazon.com/about-aws/whats-new/2016/07/amazon-elasticsearch-service-now-supports-elasticsearch-2-3/?sc_channel=sm&sc_campaign=launch_elasticsearch_service_b9abf95e&sc_publisher=tw_go&sc_content=add_support_elasticsearch_2.3&sc_country=global&sc_geo=global&sc_outcome=launches&adbsc=social_launches_20160727_64247916&adbid=758395098451161088&adbpl=tw&adbpr=66780587