chore: update webpack to 4.39.2

close #548

tested with npm test and start

package.json

@@ -89,7 +89,7 @@
     "tslint-loader": "^3.5.3",
     "typedoc": "^0.14.2",
     "typescript": "^2.6.1",
-    "webpack": "^4.25.0",
+    "webpack": "^4.39.2",
     "webpack-cli": "^3.3.2",
     "webpack-dev-server": "^3.2.1",
     "webpack-merge": "^4.1.2",