Update to reveal.js 4.6.1

Change-Id: I566737c50b93dd8c094ad3c3fa59865260b3b4ba
diff --git a/inst/reveal.js-4.6.1/plugin/chalkboard/VERSION b/inst/reveal.js-4.6.1/plugin/chalkboard/VERSION
new file mode 100644
index 0000000..cf78d5b
--- /dev/null
+++ b/inst/reveal.js-4.6.1/plugin/chalkboard/VERSION
@@ -0,0 +1 @@
+4.2.4