By the way, in the past, I have setup Travis CI to build pandoc & LaTeX projects: https://github.com/ickc/travis-ci-pandoc-latex-boilerplate/blob/master/.travis.yml#L57

you might try to follow those commands to add tlmgr in your PATH, and install latexmk. But I don't recall pandoc requires latexmk though. And the other packages in that line are what my project needed, and pandoc can do fine without them.