Fix compilation errors with pandoc-citeproc 0.4
Adjust pandoc-citeproc lower version bound.

Hakyll has been adjusted for API-incompatible change made in
pandoc-citeproc 0.4, so can't build with older versions anymore.
Merge pull request #284 from rickynils/pandoc-citeproc-0.4

Fix compilation errors with pandoc-citeproc 0.4