禁止上拉

#pragma mark - UIScrollViewDelegate
- (void)scrollViewDidScroll:(UIScrollView *)scrollView {
    // 禁止上拉
    if (scrollView.contentOffset.y >= scrollView.contentSize.height - scrollView.bounds.size.height) {
        scrollView.contentOffset = CGPointMake(0, scrollView.contentSize.height - scrollView.frame.size.height);
    }
}