Update webpack to 4.38.0 (vercel#8147)

Author: felixmosh

packages/next/package.json

@@ -97,7 +97,7 @@
     "unfetch": "4.1.0",
     "url": "0.11.0",
     "watchpack": "2.0.0-beta.5",
-    "webpack": "4.35.3",
+    "webpack": "4.38.0",
     "webpack-dev-middleware": "3.7.0",
     "webpack-hot-middleware": "2.25.0",
     "webpack-sources": "1.3.0",