Update webpack to v2.6.0 in react-scripts (#2349)

Author: ingro

packages/react-scripts/package.json

@@ -53,7 +53,7 @@
     "style-loader": "0.17.0",
     "sw-precache-webpack-plugin": "0.9.1",
     "url-loader": "0.5.8",
-    "webpack": "2.5.1",
+    "webpack": "2.6.0",
     "webpack-dev-server": "2.4.5",
     "webpack-manifest-plugin": "1.1.0",
     "whatwg-fetch": "2.0.3"