Changed iOS6 check for performance and convenience reasons

When using MarqueeLabel in UITableViewCell, the iOS6 check slows down scrolling
<code>if([[[UIDevice currentDevice] systemVersion] floatValue] >= 6.0)</code>
this way is also not 100% safe to check with. I recommend this way:
<code>if ([self.subLabel respondsToSelector:@selector(attributedText)])</code>

This pushes performance a little bit.

Steve