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](https://github.com/caolan/async/compare/v2.6.1...v2.6.4)

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

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

package.json

@@ -20,7 +20,7 @@
   },
   "main": "./lib/index.js",
   "dependencies": {
-    "async": "2.6.1",
+    "async": "2.6.4",
     "clean-css": "4.2.1",
     "color-diff": "1.4.0",
     "css-mq-parser": "0.0.3",