I bet that was it! And I somehow didn’t think to use the 17.04 repo. I’ll feel a little less silly, so that’s a big help too. 
@techAPJ, if I get ambitious, I’ll see if I can find an elegant way to add this to https://raw.githubusercontent.com/techAPJ/install-rails/master/linux. My goal is to have my new_discourse_database script to not only wipe attachments and make a new database with my admin account, but also install the development environment and create databases owned by the current user if it’s not already installed. I think that if I fix the PSQL version problem it’s done.