IPhone : SDWebImage not downloading images in iOS 7

I am using SDWebImage to download image in UITableViewCell. Its working as expected in iOS8 but its not working in iOS 7. It always showing place holder image in iOS7. If i copy the URL in browser, there its showing image. Here is my code:

[self.imageView sd_setImageWithURL:[NSURL URLWithString:@"Image download URL"] placeholderImage:[UIImage imageNamed:@"placeholder.png"]];

I am used pods to add framework.