Travis has nothing to do with this command. It’s your builder logic that generates it.
Specifically, the /bin/sh -c part ultimately comes from https://github.com/conan-io/conan-package-tools/blob/develop/cpt/packager.py#L221 .
From what I can see, you can add builder.docker_shell = "/bin/bash -c" to setup.py after the builder = line to override that value – though I didn’t test it, nor do I know if that’s the intended or even a supported way.
You should consult the authors of the builder libraries you’re using on this matter.