Enforce NPM >= 5.2

[ianpartridge]

Once #5 lands and we are using npx, we should specify that we require NPM >= 5.2 in the package.json:

{ "engines" : { "npm" : ">=5.2.0" } }

Documentation.

[ianpartridge]

Researching this further, it seems that specifying a version of NPM in "engines" is for which version of NPM can install a package, not which version of NPM a package requires at runtime.

We are probably better off using something like https://www.npmjs.com/package/check-node-version or just trying to run npx and giving a good error message if that fails.

[ianpartridge]

In c96c2ec I took the approach of "suck it and see". Try and run npx, and if that fails print an error message advising NPM >= 5.2.