I just hit this today upgrading stable.
In case it is helpful to anyone or someone can suggest a better way, this is my temporary solution:
hooks:
after_code:
- exec:
cd: $home/plugins
cmd:
- mkdir -p plugins
- git clone https://github.com/discourse/discourse-voting.git
- cd discourse-voting && git reset --hard 9b4f233b9d33a946f626597aaf3863d2305b8809 && cd ..