host our own version of remark.js

This includes a fix for code span highlighting. For details:

gnab/remark#703

Author: jdtournier

_layouts/presentation.html

@@ -99,7 +99,7 @@
     <textarea id="source">
 {{ page.content }}
     </textarea>
-    <script src="https://jdtournier.github.io/remark.min.js">
+    <script src="assets/js/remark.min.js">
     </script>
     <script>
       remark.macros.scale = function (percentage) {