Gem::LoadError: Specified 'postgresql' for database adapter, but the gem is not loaded. Add `gem 'pg'` to your Gemfile (and ensure its version is at the minimum required by ActiveRecord).
Gem::LoadError: can't activate pg (~> 0.18), already activated pg-1.0.0. Make sure all dependencies are added to Gemfile.
The second one turned out to be the main issue.
A solution was found in a rails issue 31669, briefly
Thank you for the issue. Rails still don't support pg 1.0.
My solution was to change the Gemfile reference to
group :production do
gem 'pg', '~> 0.18'
gem 'rails_12factor'
end