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