This. If I had a cloud hosted CI tool, I would gladly allow open source projects to use it. But they would be lowest priority and there would be limitations on

This. If I had a cloud hosted CI tool, I would gladly allow open source projects to use it. But they would be lowest priority and there would be limitations on how often builds could happen and/or volume of contributions. Large corporate backed opensource projects should absolutely be paying for tools like Travis CI. Just because it's open source doesn't mean there isn't money floating around. It's not super hard to rent a VPS or Physical Server somewhere and spend a few hours setting up <insert ci tool of your choice).