Update to latest version of PostCSS, v6

[maxmilton]

I'm unable to run the postcss-debug via the CLI due to the outdated version of PostCSS. Running via the Node API still works but it throws a deprecated warning.

[andywer]

Hey @maxmilton, thanks for reporting!

Do you have any time to open a PR?
I just recognized we don't have any integration tests in place (😕) either. If you could open a PR that makes it compatible with PostCSS 6 or adds an integration test, that would be very helpful!