Try this instead:
As discourse user, add to end of /var/www/discourse/Gemfile:
gem 'mysql2'
gem 'ruby-bbcode-to-md', path: '/tmp/ruby-bbcode-to-md'
As root:
cd /var/www/discourse
sudo -E -u discourse bundle install --no-deployment --path vendor/bundle
As discourse user:
cd /var/www/discourse
RAILS_ENV=production bundle exec ruby script/import_scripts/phpbb3.rb bbcode-to-md