Import rmarkdown dev version

2.3.9, the one after merging rstudio/rmarkdown#1899

Author: cderv

DESCRIPTION

@@ -61,7 +61,7 @@ License: GPL-3
 Imports:
     htmltools (>= 0.3.6),
     knitr (>= 1.22),
-    rmarkdown (>= 2.3.5),
+    rmarkdown (>= 2.3.9),
     xfun (>= 0.13),
     tinytex (>= 0.12),
     yaml (>= 2.1.19)
@@ -81,4 +81,4 @@ SystemRequirements: Pandoc (>= 1.17.2)
 LazyData: TRUE
 RoxygenNote: 7.1.1
 Encoding: UTF-8
-Remotes: rstudio/rmarkdown#1899
+Remotes: rstudio/rmarkdown