Revert "Revert "haskell-pandoc-citeproc: drop obsolete versions 0.4 and 0.5"".

This reverts commit dddea7f0d1ab765f. @jwiegley, Hakyll builds just fine with
pandoc-citeproc 0.6. See http://hydra.cryp.to/build/239248, for example. If you
have trouble compiling that package, then please share a build log that shows
the error message instead of reverting my commits, please.