make it so that mathplayer message only appears when mathjax is on the p...

...age

Couple fixups to https://github.com/edx/edx-platform/pull/1002

1) I made a mistake with the syntax on the IE conditionals

2) I realized that I only wanted the mathplayer browser message to appear when the browser wasn't compatible AND there was mathjax on the page.

Thanks @talbs and @singingwolfboy