fix(package): update async to version 2.6.1

Closes #36

Author: greenkeeper[bot]

package.json

@@ -23,7 +23,7 @@
     "commitmsg": "validate-commit-msg"
   },
   "dependencies": {
-    "async": "2.5.0",
+    "async": "2.6.1",
     "debug": "2.6.8",
     "lodash": "4.17.4"
   },