use System.Process not System.Cmd to fix 7.8.3 warning

I've tested this change on 7.6.3 & 7.8.3 and it compiles successfully.

cmdtheline is used by purescript and I want to install it via nixpkgs.  The -Werror flag in the cmdtheline.cabal ghc-options makes it fail with 7.8.3 because of a warning.  Fixing the code for the warning given (System.Process vs System.Cmd) makes it work again on both 7.6.3 & 7.8.3

Would you merge this & bump the hackage package ?