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

Start with the discussion in PR #164 and review the linked haskell-CI workaround and Cabal issue #3883. The task concerns the generated .travis.yml and is currently waiting for the Cabal issue to be resolved; done means the generated cabal invocation includes -Werror without relying on the workaround.

Written by the indexing model from the issue text.