[HOpenGL] Haddock documentation on Travis CI

I'd like to build/test the Haddock documentation for the OpenGL packages on Travis CI a bit more, so I've tweaked the configuration a bit by building the Haddock documentation for the dependencies first. One problem remains: There seems to be no installed Haddock documentation for the Haskell platform itself on Travis CI, so we get tons of warnings, e.g. here: https://travis-ci.org/haskell-opengl/GLUT Does anybody have a clue how to fix this (probably via some apt-get magic in the before-install section)? Cheers, S.