修改Tarbbar顶部横线

 CGRect rect = CGRectMake(0, 0, SCREEN_WIDTH, 0.5);
    UIGraphicsBeginImageContext(rect.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context,RGB(231, 231, 231).CGColor);
    CGContextFillRect(context, rect);
    UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [self.tabBar setShadowImage:img];
    [self.tabBar setBackgroundImage:[[UIImage alloc]init]];