Update async requirement from ~0.2.9 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@0.2.9...v2.6.3)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

package.json

@@ -21,7 +21,7 @@
     "url": "https://github.com/Clever/redis-reservation/issues"
   },
   "dependencies": {
-    "async": "~0.2.9",
+    "async": "~2.6.3",
     "debug": "~0.7.2",
     "kayvee": "^3.8.2",
     "redis": "git+https://github.com/Clever/node_redis#v0.10.1+keepalive",