fix(package): update async to version 3.1.0

Closes #303

Author: greenkeeper[bot]

package.json

@@ -9,7 +9,7 @@
     "test-server": "mocha --exit tests/server/**"
   },
   "dependencies": {
-    "async": "2.6.0",
+    "async": "3.1.0",
     "atob": "^2.0.3",
     "auth0-lock": "11.6.1",
     "babel-polyfill": "^6.26.0",