Update webpack to 4.39.1

Author: renovate-bot

package.json

@@ -98,7 +98,7 @@
     "rimraf": "2.6.3",
     "supertest": "4.0.2",
     "supertest-session": "4.0.0",
-    "webpack": "4.35.3",
+    "webpack": "4.39.1",
     "webpack-cli": "3.3.4"
   }
 }