update mardown package

[CVandiedonck]

In Rstudio, if one want to create an .Rmd file or a R notebook, the current environment requires to manually update the markdown package. The version which was installed today on my home is markdown_1.3.tar (available at : https://cran.rstudio.com/src/contrib/markdown_1.3.tar.gz)
Il would be nice to update the template to have this version automatically installed when building the environment.