chore(package): update postcss-discard-comments to version 4.0.1

Closes #74

Author: greenkeeper[bot]

package.json

@@ -101,7 +101,7 @@
     "mocha": "^3.2.0",
     "postcss-cli": "^2.6.0",
     "postcss-custom-properties": "^5.0.1",
-    "postcss-discard-comments": "^2.0.4",
+    "postcss-discard-comments": "^4.0.1",
     "postcss-importantly": "^0.0.7",
     "postcss-reporter": "^3.0.0",
     "stylelint": "^7.7.0",