build(npm): only support node versions supported by Node.js

[sverweij]

Description

• de-support node 6, 7, < 8.10, 9 and 11 (BREAKING)
• adapt the de-support error message to reflect this
• kick node 6 from all ci integration build matrices
• replace Object.assigns with object spread where this improves legibility
• ⬆️ external dependencies frozen on their last node 6 version to their latest && greatest:
  indent-string, semver-try-require, wrap-ansi, eslint, eslint-plugin-node, symlink-dir, upem

Motivation and Context

• nodejs.org still supports version 8, 10 and 12 - and dependency-cruiser will follow that.
• In addition to this, according to nodes download stats 7, 9 and 11 (source) of the last three months these versions are not used much either:

• the minimum requirement of node 8.10 is mainly to make sure we can use a builtin for determining what a core module is instead of having to rely on an external module. My assumption is that if you're still on node 8, you use latest anyway, so I expect this to impact nobody.
• Many external dependencies dependency-cruiser depends upon don't support node 6 anymore, so supporting node 6 would become problematic in the near future.

How Has This Been Tested?

• automated non-regression tests

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• The code I add will be subject to The MIT license, and I'm OK with that.
• The code I've added is my own original work.
• My code follows the code style of this project.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codeclimate]

Code Climate has analyzed commit 4865766 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (99% is the threshold).

This pull request will bring the total coverage in the repository to 99.9% (0.0% change).

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