Bump async from 2.6.1 to 2.6.4

Bumps [async](https://github.com/caolan/async) from 2.6.1 to 2.6.4.
- [Release notes](https://github.com/caolan/async/releases)
- [Changelog](https://github.com/caolan/async/blob/v2.6.4/CHANGELOG.md)
- [Commits](caolan/async@v2.6.1...v2.6.4)

---
updated-dependencies:
- dependency-name: async
  dependency-type: direct:development
...

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

Author: dependabot[bot]

package-lock.json

@@ -31,7 +31,7 @@
     "when": "~3.7"
   },
   "devDependencies": {
-    "async": "^2.0.1",
+    "async": "^2.6.4",
     "babel-eslint": "^8.2.6",
     "chai": "*",
     "debug": "~3.1.0",