travis: Install Cabal library before anything else.

I'm hoping this fixes the issue with travis
builds with earlier ghc versions.

See https://github.com/haskell/cabal/issues/4456

.travis.yml

@@ -117,6 +117,7 @@ install:
     cabal)
       cabal --version
       travis_retry cabal update
+      cabal install Cabal
       cabal install --disable-optimization --only-dependencies --flags="$FLAGS" --enable-tests --force-reinstalls --reorder-goals --max-backjumps=-1 $CABALARGS
       ;;
   esac