Select schedules based on "publication_id"

Issue Summary

views_scheduler_select_schedules() should use the "publication_id" field instead of the "schedule_id" field.

Don't ask me why, but that's how the rest of the code works. I am puzzled why "publication_id" is used like a primary key in the code, and yet the database does not describe it as such.

Without this patch, cron jobs will probably have no effect.