chore(package): update postcss-cssnext to version 3.1.0

Closes #6

Author: greenkeeper[bot]

package.json

@@ -31,7 +31,7 @@
     "http-server": "^0.9.0",
     "npmpub": "^3.1.0",
     "postcss-cli": "^2.6.0",
-    "postcss-cssnext": "^2.9.0",
+    "postcss-cssnext": "^3.1.0",
     "postcss-import": "^9.0.0",
     "stylelint": "^7.7.0",
     "stylelint-config-ticketfly": "^0.1.0"