chore(package): update broccoli-merge-trees to version 4.2.0

Closes #82

Author: greenkeeper[bot]

package.json

@@ -15,7 +15,7 @@
     "broccoli-concat": "^3.2.2",
     "broccoli-debug": "^0.6.4",
     "broccoli-funnel": "^2.0.1",
-    "broccoli-merge-trees": "^3.0.0",
+    "broccoli-merge-trees": "^4.2.0",
     "broccoli-persistent-filter": "^2.0.0",
     "broccoli-rollup": "^2.0.0",
     "broccoli-tee": "^1.0.2",