chore(package): update bluebird to version 3.1.4

http://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -16,7 +16,7 @@
     "stream-to-array": "2"
   },
   "devDependencies": {
-    "bluebird": "2",
+    "bluebird": "3.1.4",
     "mocha": "2",
     "mongodb": "2",
     "istanbul": "0"