Bump webpack-dev-server version (#240)

Author: pirropirro

package.json

@@ -49,7 +49,7 @@
     "rollup-watch": "^4.3.1",
     "webpack": "^4.39.3",
     "webpack-cli": "^3.3.10",
-    "webpack-dev-server": "^3.7.2"
+    "webpack-dev-server": "^3.9.0"
   },
   "scripts": {
     "test": "",
@@ -90,4 +90,4 @@
     "url": "https://github.com/arduino/arduino-create-agent-js-client/issues"
   },
   "homepage": "https://github.com/arduino/arduino-create-agent-js-client#readme"
-}
+}