Force failure of a Travis build if GHC produces warnings

Pandoc should, as stated in CONTRIBUTING.md, always compile without
warnings.  This patch forces GHC to treat warnings like errors during
Travis builds, making it possible to catch mistakes (like missing
top-level type definitions) early.
Merge pull request #1228 from tarleb/travis-fail-on-warnings

Force failure of a Travis build if GHC produces warnings