Change the way elements are given focus.

I know this might very much be an edge case, but I'm currently trying to use a custom build of jQuery without the `event-alias` module (among others), and that drops the `focus` method, so changing to `trigger( 'focus' )` solves that issue.

It's a non-destructive change that will make the lives of a few of us (picky) devs a little bit easier :)