Update async requirement from ^0.9.0 to ^2.6.3

Updates the requirements on [async](https://github.com/caolan/async) to permit the latest version.
- [Release notes](https://github.com/caolan/async/releases)
- [Changelog](https://github.com/caolan/async/blob/v2.6.3/CHANGELOG.md)
- [Commits](caolan/async@0.9.0...v2.6.3)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

package.json

@@ -26,7 +26,7 @@
     "url": "https://raw.github.com/hexojs/hexo-migrator-rss/master/LICENSE"
   },
   "dependencies": {
-    "async": "^0.9.0",
+    "async": "^2.6.3",
     "chai": "^3.4.1",
     "feedparser": "0.16.6",
     "request": "^2.36.0",