fix(package): update webpack-merge to version 4.0.0

Closes #105

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -82,7 +82,7 @@
     "transform-loader": "^0.2.3",
     "uglify-js": "github:mishoo/UglifyJS2#harmony",
     "webpack": "^2.1.0-beta.28",
-    "webpack-merge": "^2.5.0"
+    "webpack-merge": "^4.0.0"
   },
   "repository": {
     "type": "git",