menu-toggle.js errors out in IE 8

The menu-toggle.js uses the nth-of-type that jQuery doesn't support until 1.9 (or using JQuery-Extended-Selectors - http://stackoverflow.com/questions/11531070/nth-of-type-alternative-for-ie8) - there is also another issue with "query" being an empty object, that I didn't fully diagnose.

Given that no active use mobile devices use IE8, and it would be quite a bit of work to mangle the markup such that we don't need that selector, it seems like the best approach may simply be to return early in this case. Ideally we would use feature detection, but I couldn't find a way to do that with selectors. I don't think we can use IE conditional comments either, since this is an exclusion rather than an addition. Hence the attached patch does a dumb but working version check.