Upgrade webpack-dev-server (#2806)

Author: christianbundy

packages/react-scripts/package.json

@@ -48,7 +48,7 @@
     "sw-precache-webpack-plugin": "0.11.3",
     "url-loader": "0.5.9",
     "webpack": "2.6.1",
-    "webpack-dev-server": "2.5.0",
+    "webpack-dev-server": "2.5.1",
     "webpack-manifest-plugin": "1.1.0",
     "whatwg-fetch": "2.0.3"
   },