fix(package): update async to version 3.1.1

Closes #29

Author: greenkeeper[bot]

package.json

@@ -47,7 +47,7 @@
   "gypfile": true,
   "readmeFilename": "README.md",
   "dependencies": {
-    "async": "^2.5.0",
+    "async": "^3.1.1",
     "nan": "^2.6.2"
   },
   "devDependencies": {