Travis: I thought I didn't need alex and happy.

.travis.yml

@@ -37,7 +37,7 @@ install:
  - cabal --version
  - echo "$(ghc --version) [$(ghc --print-project-git-commit-id 2> /dev/null || echo '?')]"
  - travis_retry cabal update
- - cabal install cabal-src cabal-meta
+ - cabal install cabal-src cabal-meta alex happy
 
 # Here starts the actual work to be performed for the package under test; any command which exits with a non-zero exit code causes the build to fail.
 script: