FIX: BannerView permanently hidden after showAd(false) on IOS #67

Re-enable check for hidden bannerView.

Testing:
  IOS7 on iPhone5
  IOS7 on iPad3
  IOS6 on iPhone 3GS
Merge pull request #68 from jmelvin/master

FIX: BannerView permanently hidden after showAd(false) on IOS #67, thanks to jmelvin