Fix release-osx ERROR

[k5cents]

Per the CRAN checks, the package produces an ERROR and a related WARNING on r-release-osx-x86_64 :

The ERROR is the same as in #16 (re. pandoc).

The WARNING is new, but also stems from the lack of pandoc...

checking re-building of vignette outputs ... [6s/17s] WARNING
Error(s) in re-building vignettes:
--- re-building ‘github-spec.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
  Pandoc (>= 1.12.3) and/or pandoc-citeproc not available. Falling back to R Markdown v1.

This seems to be the same WARNING addressed in rstudio/DT#395. Some CRAN computers can't install Pandoc? Will need to look into conditional use... or remove md_define()...

[k5cents]

Also causes the same ERROR on r-oldrel-osx-x86_64.