chore(package): update webpack-cli to version 3.3.9

Closes #341

Author: greenkeeper[bot]

package.json

@@ -70,7 +70,7 @@
     "wct-browser-legacy": "1.0.2",
     "web-component-tester": "6.9.2",
     "webpack": "4.29.5",
-    "webpack-cli": "3.2.3",
+    "webpack-cli": "3.3.9",
     "webpack-dev-server": "3.1.14",
     "workbox-webpack-plugin": "3.6.3",
     "xo": "0.24.0"