Quick travis fail fix: leds_toggle()

Pull request https://github.com/contiki-os/contiki/pull/511 updated the `leds` API, but was based on an earlier version of the code than the current master, so the pull request didn't change all instances of `leds_invert()`. 

This pull request fixes the last remaining `leds_invert()` to `leds_toggle()` and should make travis happy again.