haskellPackages.threepennyGui: Fix by requiring aeson-0.6.2.1

This package did not build for me, as the cabal file has `build-depends: aeson == 0.6.*` and the NixPkgs `aeson` version is a more recent 0.7.* one.  I tried adding `jailbreak = true` to the `threepennyGui` Nix expression, but it looks like the old(er) version of `aeson` is in fact required (the interface changed), so I added a Nix expression for the old version and changed `threepennyGui` to depend on that specific version.