fix(package): update broccoli-postcss-single to version 4.0.1

Closes #83

Author: greenkeeper[bot]

package.json

@@ -43,7 +43,7 @@
   "dependencies": {
     "broccoli-funnel": "^1.1.0",
     "broccoli-merge-trees": "^1.2.1",
-    "broccoli-postcss-single": "^1.2.1",
+    "broccoli-postcss-single": "^4.0.1",
     "postcss-cssnext": "^2.9.0",
     "postcss-import": "^9.1.0",
     "ticketfly-css-box-objects": "^0.1.1",