chore(package): update ipfs-merkle-dag to version 0.4.0

http://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -61,7 +61,7 @@
     "block-stream2": "^1.1.0",
     "debug": "^2.2.0",
     "ipfs-blocks": "^0.1.2",
-    "ipfs-merkle-dag": "^0.3.0",
+    "ipfs-merkle-dag": "^0.4.0",
     "ipfs-unixfs": "^0.1.0",
     "through2": "^2.0.0"
   }