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 just posting it here in case others have a need for it, and also to start a discuss 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 this answer that one of them just doesn't work until their respective authors upgrade them to work together again?  Or do we maintain hacks like the one in this PR to allow Nix users to install both packages in the interim?  I'm not sure, since this is certainly some ugly territory I had to venture into here.

Pinging @cstrahan @peti and @shlevy