Add haddock to travis CI

[mgsloan]

I attempted to add haddock to CI, in this commit, however, the failure doesn't cause the test to fail: https://travis-ci.org/commercialhaskell/stack/jobs/100989663#L1035

I'm guessing this is a bug with cabal haddock, where it doesn't return a failing exit code. I haven't yet tried locally with cabal. stack haddock does yield an exit code when haddock fails.

[borsboom]

Why not add --haddock --no-haddock-deps to the stack test step instead of doing cabal haddock?

[mgsloan]

Ahah, good point!