Add Elasticsearch version compatibility
Add Elasticsearch version compatibility

nickstenning/collectd-elasticsearch,signalfuse/collectd-elasticsearch,signalfx/collectd-elasticsearch,sfroment/collectd-elasticsearch,F4-Group/collectd-elasticsearch,botify-labs/collectd-elasticsearch,F4-Group/collectd-elasticsearch,phobos182/collectd-elasticsearch,signalfuse/collectd-elasticsearch,Pryz/collectd-elasticsearch,signalfx/collectd-elasticsearch,botify-labs/collectd-elasticsearch,sfroment/collectd-elasticsearch,signalfx/collectd-elasticsearch