travis: document why we use hvr-ghc not travis: haskell and ghc: array with versions

[petertseng]

I submitted #121 to check why we do not use travis: haskell and ghc: to specify our GHC versions like https://docs.travis-ci.com/user/languages/haskell suggests and instead we use https://github.com/hvr/multi-ghc-travis. The reason is travis doesn't yet support 7.10 and 8.0; that's a dealbreaker for me.

To prevent any further questions, and leave notes for when we can or cannot potentially move to the travis: haskell model, we should document in the .travis.yml why it is the case that we do what we do, so that future people don't have to ask the same question I just asked.