chore(package): update async to version 2.1.5

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -24,7 +24,7 @@
   "author": "[email protected]",
   "license": "BSD",
   "dependencies": {
-    "async": "1.4.2",
+    "async": "2.1.5",
     "bluebird": "3.4.7",
     "body-parser": "1.16.1",
     "colors": "1.0.3",