fix(package): update async to version 3.1.0

Closes #32

Author: greenkeeper[bot]

package.json

@@ -32,7 +32,7 @@
     "escomplex"
   ],
   "dependencies": {
-    "async": "^2.1.5",
+    "async": "^3.1.0",
     "check-types": "^7.1.5",
     "commander": "^2.9.0",
     "escomplex": "2.0.0-alpha",