Don't complete when (region-active-p).

Hi,

Maybe it's a bit contrived to check if the region is active,
but I'm pretty sure no one would want to auto-complete in that case.

My use case is that I've bound "m" to a command that either self-inserts or marks sexp.
I've added it to `ac-trigger-commands` because I want to auto-complete after self-inserting,
but I don't want to auto-complete after marking.