From time to time I see PR that fixes composer.json for concrete component.
I assume that these failures are happening because each component is not tested by Travis CI as standalone project.
Thus missed composer dependencies in concrete component are usually found by people, not Travis CI.
Maybe each component has to have own .travis.yml and Travis CI should test each component via relevant mirror repository?

P.S. Feel free to close this issue if you think that it is not worth it. Just suggestion.