Change needed for “virtual coupons”

The function woocoupon_schedule_check_validity() needs to check if the coupon is virtual. Virtual coupons doesn’t exist in the database so get_post_meta will fail and this plugin will incorrectly say that a valid virtual coupon is invalid.

Adding

if ( $coupon->get_virtual() ) {
    return $valid;
}

at the beginning of woocoupon_schedule_check_validity() would fix this problem.