You can use Django to dump data from your db and git to push the dump to heroku. If the data is sensitive you should take care to remove the data by resetting the head after loading the data in heroku. A more in depth solution can be found here.

1. Dumpdata from dev build.

python manage.py dumpdata --natural --indent 2 > data.json

2. Add data to repo and commit.

git add data.json  git commit -m "Dump data"

3. Push to Heroku.

git push heroku master

4. Load data into Heroku build.

heroku run python manage.py loaddata data.json