Upgrade to ghc 7.8.3 and cabal 1.20.0.3

Upgrades to the latest ghc / cabal.

Thanks. Can you do it like [this](https://github.com/mietek/haskell-on-heroku/commit/ffb59c86e11f6c493d24b38245d9ae1ce2caf46d#diff-984d1cfb4fbf6bfb7a3bb7a3b08f8292R426)?