Change to event delegation for ef click event

I had a problem in WP 3.9.2 while using chrome where the checkbox selector was running incredibly slowly and incorrectly adding event handlers to the extent that the post list admin pages were unusable for up to 30 seconds.

![html-parse-timings](https://cloud.githubusercontent.com/assets/23417/4493576/834ab3f4-4a4a-11e4-8769-1e5f00ad79cf.png)

Changing line 9 to use `.on()` fixed it.

Possibly to do with changes in the sizzle library but this fixes it.