chore(package): update postcss-custom-properties to version 8.0.4

Closes #74

Author: greenkeeper[bot]

package.json

@@ -69,7 +69,7 @@
     "moment": "2.18.1",
     "moment-timezone": "0.5.13",
     "postcss-color-function": "^4.0.0",
-    "postcss-custom-properties": "^6.0.1",
+    "postcss-custom-properties": "^8.0.4",
     "postcss-easy-import": "^2.0.0"
   }
 }