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

Change-Id: I6384639329daef18a40ca55cbd3c4eab0ae500e1
diff --git a/.Rbuildignore b/.Rbuildignore
index a85365b..2d8fc9a 100644
--- a/.Rbuildignore
+++ b/.Rbuildignore
@@ -12,6 +12,7 @@
 ^tests/manual$
 ^tools$
 
+^LICENSE\.md$
 inst/reveal.js-[^/]+/.github
 inst/reveal.js-[^/]+/.gitignore
 inst/reveal.js-[^/]+/.npmignore
@@ -24,4 +25,3 @@
 inst/reveal.js-[^/]+/package-lock.json
 inst/reveal.js-[^/]+/package.json
 inst/reveal.js-[^/]+/test
-^LICENSE\.md$