Make further use of Travis matrices.

Created by: lwm

Currently, different GHC versions are in different matrices of the Travis build. However, I'd like to go even further and separate the Hlint build and the Doctest build from the main GHC related builds. This means I can get faster feedback from a build run with fast_finish: true as well.

Oh, I should also add a haddock build. Well, that can come later (EDIT: See https://github.com/lwm/tasty-discover/issues/45).