add engines config

[givanse]

What Node versions does this npm module support?

I might be wrong, but I'm guessing >= 4. The reason is Node 0.10, 0.11 and 0.12 have reached end-of-life.

[fb55]

The underlying networking library is request, as long as they support a node version this library should continue to work as well. (Of course newer versions of node are also supported.)

[givanse]

https://github.com/request/request/blob/c57fb7258788888e42fcfa2b6d54c8150517cabe/package.json

  "engines": {
    "node": ">= 4"
  },

this is great news, most of ES6 could be used ;)

[awitherow]

Who is in charge of this project? Can this be merged?

[awitherow]

@fb55 any ideas on getting this merged with a higher node verison? Would automagically help a lot of "modernized" failed tests.

[nikulis]

Of interest would be Node's official LTS schedule.

With 4.x reaching end-of-life in less than a year, and 8.x reaching official LTS even sooner than that, I think that going with any of 6.x - 8.x would be an ok move.

In my own opinion, because I love async/await so much, I would recommend >= 7.6.0.

[fb55]

I merged this to already have a basis for specifying a version. I would like to still support one LTS release, which means we should support node 6. Please let me know if there are any reasons besides async/await* to specify any version higher than 6.0.0.

* I'm also a fan of async/await, but having one battle-tested version of node support this library has precedence for now.