Because we’re mounting the plugin code as a volume in the docker container, it gets all of its permissions/owner/group from the host system (travis). The UID of the travis user, and the UID of the “discourse” user inside the container are not the same, and so the discourse container does not have write access to the plugin directory.
I would never recommend this in a production system, but since this is just tests, I think it is safe to do a chmod -R 777 on the plugin files before running the tests. I wouldn’t call it a fix, but it does work around the problem.
I submitted a PR to your repo with the 1 line fix, which works for travis builds on my fork 
I’ll add a note to the OP here in case anyone else has the same issue