Use PostCSS 7

[ai]

There are no real breaking changes, we just dropped Node.js 4

[ai]

/cc @michael-ciniawsky

[damanwiththeplan]

made a pull request

[michael-ciniawsky]

I need to make a quick check on the weekend what else might be a good idea to potentionally include in the next a major release + the additional chores like docs polishing etc. I don't think there are other (needed) breaking changes atm, so I'm heading to cut a release on the weekend/monday for PostCSS v7.0.0 (#375). If there is anything else to triage please open an issue