Update chakra default argument in moon_reader

To revert temporarily back to the old version of remark.js (prior to 0.15.0) in order to fix issue #245 

If we decide to go forward with this PR I suggest putting in a milestone so we remember to update the reference such that it points to the most recent version of remark.js once that has stabilized again with the new maintainer.

R/render.R

@@ -71,7 +71,7 @@
 #' # rmarkdown::render('foo.Rmd', 'xaringan::moon_reader')
 moon_reader = function(
   css = c('default', 'default-fonts'), self_contained = FALSE, seal = TRUE, yolo = FALSE,
-  chakra = 'https://remarkjs.com/downloads/remark-latest.min.js', nature = list(),
+  chakra = 'https://cdnjs.cloudflare.com/ajax/libs/remark/0.14.0/remark.min.js', nature = list(),
   ...
 ) {
   theme = grep('[.]css$', css, value = TRUE, invert = TRUE)