Upgrade PostCSS dependencies

[StarpTech]

They are outdated https://github.com/css-modules/css-modules-loader-core/blob/master/package.json#L11-L15

This is the first step to upgrade this package in postcss-modules.

I opened this issue to solve #235

[lukeed]

Analysis/breakdown here: #231 (comment)

I fear no one cares.