Add support for Elasticsearch 6

[zusrsoft]

Can you support the ES6.x version, thank you. Please upgrade.

[hatdropper1977]

To the devs - is this on the roadmap?

[snoopy83101]

I'm waiting for it, too

[a-magdy]

I am not sure if it is fully supported but the major change is in mapping types, and you can use rename namespace to send each collection to its own index, check my answer here: https://github.com/mongodb-labs/elastic2-doc-manager/issues/57#issuecomment-371524332

[tigerking]

a workaround, pls check if it works for you #60