fix(package): update async to version 3.1.1

Closes #38

Author: greenkeeper[bot]

package.json

@@ -10,7 +10,7 @@
     "build": "babel lib -d build"
   },
   "dependencies": {
-    "async": "^2.1.4"
+    "async": "^3.1.1"
   },
   "devDependencies": {
     "babel-cli": "^6.0.0",