[HOpenGL] Haddock documentation on Travis CI

[Sven Panne]

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.