Bump firebase-admin from 6.3.0 to 6.4.0 in /functions (#10)

Bumps [firebase-admin](https://github.com/firebase/firebase-admin-node) from 6.3.0 to 6.4.0.
- [Release notes](https://github.com/firebase/firebase-admin-node/releases)
- [Changelog](https://github.com/firebase/firebase-admin-node/blob/master/CHANGELOG.md)
- [Commits](firebase/firebase-admin-node@v6.3.0...v6.4.0)

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

functions/package.json

@@ -18,7 +18,7 @@
  "main": "lib/index.js",
  "dependencies": {
  "actions-on-google": "^2.5.0",
- "firebase-admin": "^6.3.0",
+ "firebase-admin": "^6.4.0",
  "firebase-functions": "^2.1.0",
  "node-fetch": "^2.3.0",
  "prettier": "*",

functions/yarn.lock

@@ -868,9 +868,9 @@ finalhandler@1.1.1:
  statuses "~1.4.0"
  unpipe "~1.0.0"
 
-firebase-admin@^6.3.0:
- version "6.3.0"
- resolved "https://registry.yarnpkg.com/firebase-admin/-/firebase-admin-6.3.0.tgz#68ca0842ece2774c9455f7e1a054bacc24dde804"
+firebase-admin@^6.4.0:
+ version "6.4.0"
+ resolved "https://registry.yarnpkg.com/firebase-admin/-/firebase-admin-6.4.0.tgz#55c288f4e324bc89174bad39735c78475eae7235"
  dependencies:
  "@firebase/app" "^0.3.4"
  "@firebase/database" "^0.3.6"