Declare the markdown package as a dependency of the SparkR package

If we didn't install pandoc locally, the make-distribution package will fail with the following message:

— re-building ‘sparkr-vignettes.Rmd’ using rmarkdown
Warning in engine$weave(file, quiet = quiet, encoding = enc) :
Pandoc (>= 1.12.3) not available. Falling back to R Markdown v1.
Error: processing vignette 'sparkr-vignettes.Rmd' failed with diagnostics:
The 'markdown' package should be declared as a dependency of the 'SparkR' package (e.g., in the 'Suggests' field of DESCRIPTION), because the latter contains vignette(s) built with the 'markdown' package. Please see https://github.com/yihui/knitr/issues/1864 for more information.
— failed re-building ‘sparkr-vignettes.Rmd’