chore(package): update postcss-cssnext to version 3.0.1

Closes #7

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "live-server": "^1.2.0",
     "nsp": "^2.6.3",
     "postcss-cli": "^4.0.0",
-    "postcss-cssnext": "^2.11.0",
+    "postcss-cssnext": "^3.0.1",
     "postcss-import": "^10.0.0",
     "stylelint": "^7.10.0"
   },