fix(package): update webpack-merge to version 4.2.2

Closes #44

Author: greenkeeper[bot]

package.json

@@ -57,7 +57,7 @@
     "webpack": "1.13.3",
     "webpack-dev-middleware": "1.8.4",
     "webpack-hot-middleware": "2.13.1",
-    "webpack-merge": "2.1.1"
+    "webpack-merge": "4.2.2"
   },
   "peerDependencies": {
     "react": ">=0.14.0",