A very simple buildpack to convince Heroku to update your submodules during a deploy.
Heroku will normally (do this automatically)[https://devcenter.heroku.com/articles/git-submodules].
However, if you're using Heroku's Dashboard UI to deploy or using review apps, it will not.
In these cases you require a buildpack like this one.
heroku buildpacks:add https://github.com/dmathieu/heroku-buildpack-submodules