fix(if): do not warn for >10 files

gulp-if currently gives a warning if you pass >10 files through, because EventEmitter has a [default maxListeners of 10](http://nodejs.org/api/events.html#events_emitter_setmaxlisteners_n).

We can set this to unlimited by doing `setMaxListeners(0)`.

We could also refactor it so there is only one child listener, but it would be slightly more complicated.