Update minimum NodeJS and NPM versions required

[ntwb]

This module requires postcss-load-config which in turn requires postcss-load-plugins and postcss-load-options, all three of these repos have a NodeJS & NPM minimum requirement:

• "engines": { "node": ">=0.12", "npm": ">=3" },`

https://github.com/michael-ciniawsky/postcss-load-config/blob/master/package.json#L5
https://github.com/michael-ciniawsky/postcss-load-plugins/blob/master/package.json#L5
https://github.com/michael-ciniawsky/postcss-load-options/blob/master/package.json#L5

To prevent warnings when using this module this repo should also follow the same min req.

[ai]

@ntwb why we should add engines here and not to postcss-load-config?

[michael-ciniawsky]

@ai Doesn't hurt to provide metadata about node and npm version, I would say merge it 😛

@ntwb Doesnpm install --save|--save-dev postcss-loader currently display any warnings, because of missing pkg.engines prop?

[ntwb]

@ai, postcss-load-config already has engines defined: https://github.com/michael-ciniawsky/postcss-load-config/blob/master/package.json#L5

Here's the details of Travis CI job I noticed this in: https://travis-ci.org/stylelint/stylelint-demo/jobs/175820550#L172

[ai]

@ntwb so, do we need a engines in postcss-loader if it is already in postcss-load-config?

[ntwb]

@ntwb so, do we need a engines in postcss-loader if it is already in postcss-load-config?

Yes, which is what this PR fixes 😄

[ai]

@ntwb and why we need it if it is already in postcss-load-config?

[ntwb]

If a repo you depend upon requires NodeJS v7.x, then this repo should also state that the minimum requirement is NodeJS v7.x

In this case though postcss-load-config has a minimum NPM requirement for NPM v3.x, so that same minimum requirement should be included in postcss-loader

Edit: You cannot use postcss-loader if you use NPM 2.x, you must have NPM 3.x

[ai]

I mean when benefits user will get? npm warning will be the same?

[ntwb]

They will no longer get the NPM warning

[ntwb]

Thanks :)