去掉tabbar上的黑线

- (void)viewDidLoad {
    [super viewDidLoad];
    // 去掉顶部的黑线
    self.tabBar.backgroundImage = [UIImage imageWithColor:[UIColor clearColor]];
    [self.tabBar setShadowImage:[UIImage imageWithColor:[UIColor clearColor] size:CGSizeMake(kScreenWidth, 0.5)]];