Revert raising of the cabal version in the build-dependency
Revert raising of the cabal version in the build-dependency

Travis was failing because it uses `cabal install --only-dependencies`
in the .travis.yml

sdiehl/ghc,bgamari/shaking-up-ghc,sdiehl/ghc,izgzhen/hadrian,snowleopard/hadrian,sdiehl/ghc,sdiehl/ghc,snowleopard/shaking-up-ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc