Issue
You are seeing ERROR: must be owner of extension plpgsql or similar when you attempt to run rake db:structure:load.
Resolution
When Rails generates structure.sql, it includes commands to enable the database extensions it wants to use. Unfortunately, these are blocked by heroku-postgresql. One way around this is to use config.active_record.schema_format = :ruby, which means your app would generate a schema.rb instead of a structure.sql. schema.rbis compatible with heroku-postgresql because it doesn't enable extensions.
If you need to use structure.sql you can remove COMMENT ON EXTENSION plpgsql IS 'PL/pgSQL procedural language'; which should also resolve this issue.