Update to PostCSS 6.x

[ntwb]

PostCSS 6.0.6 is the latest release.

I'd created a PR in #83 to test this, Travis CI passed all 3 jobs though this was not actually the case, when inspecting the job logs all 3 had failed but still returned an exit of 0

Apologies for deleting the template, but there was not much to answer with this issue context

[shellscape]

Given the version it's on at present, that's probably bundled with a bunch of breaking changes. That's going to be a major version for this module for sure.

[marcusdarmstrong]

Would you be open to a contribution on this issue?

[shellscape]

@marcusdarmstrong surely.

[ai]

@shellscape we have PostCSS 7.0 right now. We really need to update parser, since most of the plugins support only 6-7 AST API.

If you have no time, write me, I will send people to help you.

[shellscape]

@ai happy to review PRs to update this

[ai]

@shellscape done #112