chore(package): update webpack-merge to version 4.1.5

Closes #193

Author: greenkeeper[bot]

package.json

@@ -143,7 +143,7 @@
     "typedoc": "^0.5.7",
     "typescript": "^2.2.1",
     "webpack": "^2.2.1",
-    "webpack-merge": "^3.0.0"
+    "webpack-merge": "^4.1.5"
   },
   "babel": {
     "env": {