haskell-hakyll: allow building hakyll with GHC 7.6.3

OK, this pull request is pretty horrible and I'd rather never see it merged.  I'm posting it here in case others have a need for it, and also to start a discussion on how we intend to deal with packages that break, like `hakyll`, when we upgrade other parts of the system, in this case `pandoc`.

Is the answer that one of them just doesn't work until the respective authors upgrade them both to work together?  Or do we maintain hacks like this one to allow Nix users to install both packages in the meantime?  I'm not sure, since this is certainly some ugly territory I had to venture into here.

Pinging @cstrahan @peti and @shlevy