Travis-CI: Build alex and happy in parallel.

.travis.yml

@@ -13,8 +13,7 @@ before_install:
   - export SCARGOT_REPO=$PWD/s-cargot
 install:
   - cabal install cabal-install
-  - cabal install alex
-  - cabal install happy
+  - cabal install alex happy
 script:
   - ghc --version
   - cabal --version