add support ghc-7.8 by relaxing cabal constraint

As commented on https://github.com/tmhedberg/hsenv/issues/40, I've tried to build hsenv-0.4 with ghc-7.8.2.
ghc-7.8.2 has new version of `base`, `Cabal`, `process` and `unix` packages.
These new packages causes unsatisfy dependencies.