Use 16:9 aspect and bump reveal.js to 5.1.0

Change-Id: I6384639329daef18a40ca55cbd3c4eab0ae500e1
diff --git a/inst/reveal.js-5.1.0/plugin/chalkboard/VERSION b/inst/reveal.js-5.1.0/plugin/chalkboard/VERSION
new file mode 100644
index 0000000..df0228d
--- /dev/null
+++ b/inst/reveal.js-5.1.0/plugin/chalkboard/VERSION
@@ -0,0 +1 @@
+4.2.5