Following my request to Travis support, they have granted us 20'000 credits for open-source builds; with the 10'000 free credits that should be enough for few months.

It is worth noting that per the FAQ it is also possible to request a renewable amount now, something that was not an option a few months ago.

CI builds have been migrated from Travis-CI .org (https://travis-ci.org/github/mantisbt/) to .com (https://app.travis-ci.com/github/mantisbt/mantisbt) and our current configuration seem to be working fine.