Add `-Werror` to the `cabal` into generated `.travis.yml`

See discussion in this PR:

* https://github.com/kowainik/summoner/pull/164#issuecomment-427236215

The following workaround exists:

* https://github.com/haskell-CI/haskell-ci/issues/95

But we're waiting for this `cabal` issue to be resolved first:

* https://github.com/haskell/cabal/issues/3883
