Bump async from 2.5.0 to 2.6.4 in /generator-exercise

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: indirect
...

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

generator-exercise/package-lock.json

@@ -179,11 +179,11 @@
       "dev": true
     },
     "async": {
-      "version": "2.5.0",
+      "version": "2.6.4",
-      "resolved": "https://registry.npmjs.org/async/-/async-2.5.0.tgz",
+      "resolved": "https://registry.npmjs.org/async/-/async-2.6.4.tgz",
-      "integrity": "sha512-e+lJAJeNWuPCNyxZKOBdaJGyLGHugXVQtrAwtuAe2vhxTYxFTKE73p8JuTmdH0qdQZtDvI4dhJwjZc5zsfIsYw==",
+      "integrity": "sha512-mzo5dfJYwAn29PeiJ0zvwTo04zj8HDJj0Mn8TD7sno7q12prdbnasKJHhkm2c1LgrhlJ0teaea8860oxi51mGA==",
       "requires": {
-        "lodash": "^4.14.0"
+        "lodash": "^4.17.14"
       }
     },
     "asynckit": {