fix(package): update async to version 3.1.0

Closes #7

Author: greenkeeper[bot]

package.json

@@ -7,7 +7,7 @@
     "packity": "bin/packity.js"
   },
   "dependencies": {
-    "async": "^2.1.4",
+    "async": "^3.1.0",
     "colors": "^1.1.2",
     "commander": "^2.9.0",
     "lodash": "^4.17.2",