Update heroku-deploy workflow: v3.12.14 -> v3.13.15

cf. https://github.com/AkhileshNS/heroku-deploy/releases

Author: yasulab

.github/workflows/test.yml

@@ -54,7 +54,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v3
-      - uses: akhileshns/heroku-deploy@v3.12.14
+      - uses: akhileshns/heroku-deploy@v3.13.15
         with:
           heroku_api_key:  ${{ secrets.HEROKU_API_KEY }}
           heroku_app_name: ${{ secrets.HEROKU_APP_NAME }}