feat: upgrade async from 2.6.1 to 3.2.4

Snyk has created this PR to upgrade async from 2.6.1 to 3.2.4.

See this package in npm:
https://www.npmjs.com/package/async

See this project in Snyk:
https://app.snyk.io/org/contentstack-cli/project/48e01156-bda4-4b15-ae2f-09e8636b982c?utm_source=github&utm_medium=referral&page=upgrade-pr

Author: snyk-bot

package.json

@@ -7,7 +7,7 @@
     "contentstack": "./bin/index.js"
   },
   "dependencies": {
-    "async": "2.6.1",
+    "async": "3.2.4",
     "chalk": "2.4.1",
     "commander": "2.9.0",
     "debug": "4.1.0",