Fix compilation errors with pandoc-citeproc 0.4

This fixes a compilation error in `Web/Pandoc/Biblio.hs` caused by a changed type signature of `readCSLFile` in pandoc-citeproc (https://hackage.haskell.org/package/pandoc-citeproc-0.4.0.1/docs/Text-CSL-Parser.html#v:readCSLFile), reported in issue #280. A parameter `mbLocale :: Maybe String` has been added. In my patch, I just set this parameter to `Nothing`. I do not know what consequences that may have for Hakyll.

I've also bumped the `pandoc` and `pandoc-citeproc` version bounds in `hakyll.cabal`.