chore(package): update postcss-mixins to version 6.0.0

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -79,7 +79,7 @@
     "postcss-extend": "^1.0.5",
     "postcss-for": "^2.1.1",
     "postcss-initial": "^1.5.3",
-    "postcss-mixins": "^5.4.1",
+    "postcss-mixins": "^6.0.0",
     "postcss-nested": "^1.0.0",
     "postcss-reduce-initial": "^1.0.1",
     "postcss-sorting": "^2.0.1",