update packages and add valign

scripts/reveal.js/plugin/math/plugin.js

@@ -1,15 +1,17 @@
 import {KaTeX} from "./katex";
 import {MathJax2} from "./mathjax2";
 import {MathJax3} from "./mathjax3";
+import {MathJax4} from "./mathjax4";
 
 const defaultTypesetter = MathJax2;
 
 /*!
  * This plugin is a wrapper for the MathJax2,
- * MathJax3 and KaTeX typesetter plugins.
+ * MathJax3, MathJax4 and KaTeX typesetter plugins.
  */
 export default Plugin = Object.assign( defaultTypesetter(), {
 	KaTeX,
 	MathJax2,
-	MathJax3
+	MathJax3,
+	MathJax4
 } );