chore(dependencies): update postcss to latest version

postcss was updated to version 6.

BREAKING CHANGE:
css-byebye will work from node v4 from now on.

postcss@6 drops support for nodejs 0.12. This should normally not be
a problem if you use recent versions of node, but it is a breaking
change nonetheless.

Links:
https://github.com/postcss/postcss/releases/tag/6.0.0

Author: AoDev

.travis.yml

@@ -3,5 +3,6 @@ node_js:
   - "4"
   - "5"
   - "6"
-  - "0.12"
-  - "iojs"
+  - "7"
+  - "8"
+  - "9"