updated to work with pandoc 1.12

Pandoc 1.12 decouples citeproc-hs from itself, so there is no longer a
Text.Pandoc.Biblio module in Pandoc. Further, citeproc-hs depends on
pandoc-types 1.10 but Pandoc 1.12 depends on pandoc-types 1.12. To
alleviate these issues, pandoc-citeproc was created which includes a
copy of the citeproc-hs source made to be compatible, since the
developer of citeproc-hs is apparently MIA.

pandoc-citeproc is a separate module that handles the mixture of
citeproc-hs and Pandoc. It includes `processCites` in Text.CSL.Pandoc,
which is the new name of what used to be `processBiblio` from
Text.Pandoc.Biblio

Most of these changes are seamless, consisting of simple name changes in
both functions and modules. However, a more direct change in the
Hakyll API itself is that `readPandocBiblio`'s second parameter, the
CSL, is now mandatory, i.e. not of type Maybe. This is to reflect the
same change in the underlying processing function from Text.CSL.Pandoc,
`processCites`, where the Style argument is now mandatory, and the style
is derived from the CSL.

See the old function:

    processBiblio :: Maybe Style -> [Reference] -> Pandoc -> Pandoc

Compared to the new one:

    processCites :: Style -> [Reference] -> Pandoc -> Pandoc

Sources:

* http://hackage.haskell.org/packages/archive/pandoc/1.11.1/doc/html/Text-Pandoc-Biblio.html
* http://hackage.haskell.org/packages/archive/pandoc-citeproc/0.1/doc/html/Text-CSL-Pandoc.html

Similarly, there is no longer a `readerReferences` field in the reader
options structure.
Restrict pandoc version a bit more