Fix for NSRangeException, attempting to remove observer twice.
Fix for NSRangeException, attempting to remove observer twice.

untouchable741/VGParallaxHeader,stoprocent/VGParallaxHeader,zilaiyedaren/VGParallaxHeader,phaibin/VGParallaxHeader,untouchable741/VGParallaxHeader,stoprocent/VGParallaxHeader,ajijoyo/VGParallaxHeader,ajijoyo/VGParallaxHeader,stoprocent/VGParallaxHeader,ajijoyo/VGParallaxHeader,apascual/VGParallaxHeader,zilaiyedaren/VGParallaxHeader,apascual/VGParallaxHeader,untouchable741/VGParallaxHeader,phaibin/VGParallaxHeader,achyusuf/VGParallaxHeader,achyusuf/VGParallaxHeader,phaibin/VGParallaxHeader,zilaiyedaren/VGParallaxHeader,apascual/VGParallaxHeader,achyusuf/VGParallaxHeader