Upgrade postcss-apply dependency

- Implement proper property set overrides
- See pascalduez/postcss-apply#10
- Implement from js custom property sets declaration
- See pascalduez/postcss-apply#15
- Added a `preserve` option
- See pascalduez/postcss-apply#14

Author: pascalduez

package.json

@@ -27,7 +27,7 @@
     "pixrem": "^3.0.0",
     "pleeease-filters": "^3.0.0",
     "postcss": "^5.0.4",
-    "postcss-apply": "^0.3.0",
+    "postcss-apply": "^0.6.0",
     "postcss-attribute-case-insensitive": "^1.0.1",
     "postcss-calc": "^5.0.0",
     "postcss-color-function": "^2.0.0",