Drop support for < Node 4

[voxpelli]

With reference to the test failure in #61: https://travis-ci.org/maxogden/dependency-check/jobs/196024596

Being able to use such simple things as Object.assign() makes for much simpler code. Isn't it time to stop supporting the older versions soon to enable that?

[blakeembrey]

I don't think there's any reason to upgrade prematurely. It might be worth keeping track of for a next major release.

[voxpelli]

As the failing tests in #79 showed, recently dependency-check stopped working on version older than Node 4 as read-package-json broke compatibility on one of their patch versions: npm/read-package-json#72

This has shown in other projects as well: request/request#2772 (comment)

So question is how long we can actually claim to support those versions. We should probably move ahead and drop at least Node 4 pretty soon.

I would even go as far as to say that now we should aim for Node 6 and above as Node 4 ends LTS in April so everyone should be moving away from it now anyways. Node 6 should be the oldest Node-version any actively maintained project is using right now and older projects can always use the 2.x branch

[voxpelli]

I would suggest we target Node 6 in 3.0.0 as Node 4 reaches end of life in three months – do you agree @blakeembrey?

[blakeembrey]

SGTM.

[voxpelli]

Fixed by: d0efa70