fix(package): update async to version 2.4.0

https://greenkeeper.io/

Author: greenkeeper[bot]

package.json

@@ -54,7 +54,7 @@
     "npm": ">=2.15.9 <3.0.0"
   },
   "dependencies": {
-    "async": "2.3.0",
+    "async": "2.4.0",
     "bloomrun": "2.3.0",
     "eq8-core": "1.0.7",
     "joi": "9.2.0",