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

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -66,7 +66,7 @@
     "np": "^2.13.2",
     "postcss-at-rules-variables": "0.1.0",
     "postcss-attribute-selector-prefix": "0.1.0",
-    "postcss-calc": "^5.3.1",
+    "postcss-calc": "^6.0.0",
     "postcss-class-prefix": "^0.3.0",
     "postcss-clearfix": "^1.0.0",
     "postcss-cli": "^3.2.0",