Elasticsearch 6.x support

I notice that the code doesn’t seem to be using the actual Ruby gem at this time; and that there’s an issue with the deprecation of “string” for “text”. Having trouble finding that in the code to fix; otherwise, I can imagine that the logic that detects 5.x vs 6.x usage, could be used to indicate if “string” or “text” should be used.

https://github.com/medcl/elasticsearch-proxy appears to allow Zammad to proxy requests to an ES6 instance.