markdown::markdownExtensions() removed.

[davidaarmstrong]

Since version 1.4 of the markdown() package, the markdownExtensions() function has been removed. Currently, lines 332-336 of file_strem_server.r are:

        text = markdown::markdownToHTML(
          text = text,
          fragment.only = TRUE,
          extensions = markdown::markdownExtensions()
        )