fix(package): update async to version 3.1.1

Closes #100

Author: greenkeeper[bot]

package.json

@@ -26,7 +26,7 @@
   "dependencies": {
     "@types/express": "^4.11.1",
     "@types/joi": "^14.0.0",
-    "async": "2.6.1",
+    "async": "3.1.1",
     "cookie-parser": "1.4.3",
     "debug": "^4.1.0",
     "express": "^4.16.4",