fix(package): update async to version 3.1.1

Closes #56

Author: greenkeeper[bot]

package.json

@@ -5,7 +5,7 @@
   "main": "server.js",
   "dependencies": {
     "arangojs": "^6.2.3",
-    "async": "^2.5.0",
+    "async": "^3.1.1",
     "body-parser": "^1.18.2",
     "colors": "^1.1.2",
     "cors": "^2.8.4",