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

Closes #71

Author: greenkeeper[bot]

package.json

@@ -44,7 +44,7 @@
     "broccoli-funnel": "^1.1.0",
     "broccoli-merge-trees": "^1.2.1",
     "broccoli-postcss-single": "^1.2.1",
-    "postcss-cssnext": "^2.9.0",
+    "postcss-cssnext": "^3.0.1",
     "postcss-import": "^9.1.0",
     "ticketfly-css-box-objects": "^0.1.1",
     "ticketfly-css-box-shadow-garnishes": "^0.0.1",