Make marginRight work in firefox

Summary:
`.style["margin-right"]` doesn't work in firefox, but
`.style.marginRight` does.

Test Plan:
- Look at `b\llap{f}` in chrome and firefox
- Make sure it looks the same (or at least very similar) in both

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D7476

buildTree.js

@@ -507,7 +507,7 @@ var makeText = function(value, style) {
             metrics.depth);
         if (metrics.italic > 0) {
             var span = makeSpan([], [textNode]);
-            span.style["margin-right"] = metrics.italic + "em";
+            span.style.marginRight = metrics.italic + "em";
 
             return span;
         } else {