Bump async from 2.5.0 to 2.6.4

Bumps [async](https://github.com/caolan/async) from 2.5.0 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/v2.5.0...v2.6.4)

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

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

package.json

@@ -21,7 +21,7 @@
   "homepage": "https://github.com/koodiklinikka/koodiklinikka.fi-api",
   "dependencies": {
     "apicache": "0.0.12",
-    "async": "^2.5.0",
+    "async": "^2.6.4",
     "bluebird": "^2.9.3",
     "body-parser": "^1.10.1",
     "cors": "^2.7.1",