If your deploy is installing older/outdated versions, you may need to purge your Heroku repo:
heroku plugins:install heroku-repo
heroku repo:purge_cache -a my-app
Should you encounter a segfault during builds:
bin/compile: line 603: 3547 Segmentation fault composer config --no-plugins
Try purging your build cache:
heroku plugins:install heroku-repo
heroku repo:purge_cache -a my-app