chore(package): update async to version 2.0.0

https://greenkeeper.io/

Author: greenkeeperio-bot

package.json

@@ -7,7 +7,7 @@
     "read": "^1.0.4"
   },
   "devDependencies": {
-    "async": "^1.5.1",
+    "async": "^2.0.0",
     "expect.js": "^0.3.1",
     "mocha": "^2.1.0"
   },