chore(dependencies): Bump webpack from 3.8.1 to 3.9.1

Bumps [webpack](https://github.com/webpack/webpack) from 3.8.1 to 3.9.1.
- [Release notes](https://github.com/webpack/webpack/releases)
- [Commits](webpack/webpack@v3.8.1...v3.9.1)

Author: dependabot[bot]

package-lock.json

@@ -25,7 +25,7 @@
     "source-map-loader": "^0.2.3",
     "tape": "^4.8.0",
     "typescript": "^2.6.2",
-    "webpack": "^3.8.1",
+    "webpack": "^3.9.1",
     "webpack-node-externals": "^1.6.0"
   }
 }