fix(package): update async to version 3.1.0

Closes #410

Author: greenkeeper[bot]

package.json

@@ -27,7 +27,7 @@
     ]
   },
   "dependencies": {
-    "async": "2.6.0",
+    "async": "3.1.0",
     "body-parser": "1.18.2",
     "cookie-parser": "1.4.3",
     "debug": "3.1.0",