I too faced such issues. In order to re run travis-ci, I used to make two additional commits on top of the main commit. The first commit involved a minor change and the second commit involved revert of first commit. I used to then squash the last 3 commits into one i.e., restore the repository to original state. In case, if you try to push again to the upstream after squashing, travis-ci build will be triggered automatically.

Although this might not be the best possible way, but it's at least a workaround.

Cheers,