canary - just updating to the latest stable version of webpack (#5623)

In order to trying to resolve the bug [#5480](https://github.com/zeit/next.js/issues/5480), I create this PR which update the webpack version.

packages/next/package.json

@@ -78,7 +78,7 @@
     "terser-webpack-plugin": "1.0.2",
     "unfetch": "3.0.0",
     "url": "0.11.0",
-    "webpack": "4.20.2",
+    "webpack": "4.25.1",
     "webpack-dev-middleware": "3.4.0",
     "webpack-hot-middleware": "2.22.3",
     "webpack-sources": "1.2.0",