nix-prefetch-scripts: new expression
Merge pull request #2887 from bennofs/nix-prefetch-scripts

nix-prefetch-scripts: new expression
ghc-head: update to version 7.9.20140608
haskell-defaults.nix: ghc-HEAD requires latest mtl
haskell-syb: update to version 0.4.2
haskell-syb: drop obsolete versions 0.2.2 and 0.3.6.2