Support for node versions >= v6

[russormes]

WHAT

Supporting older version of node requires not using es2025 features such as const.

Proposal

I believe we should support back to node v6 which includes all es2015 features.
http://node.green/

Anyone who intends to use this lib will have to either be refactoring or developing from scratch. There is not a great use case for building a new web service using outdated versions of node.