Remove extra padding in BannerView (#43)

Author: LutherDiaz

src/ios/CTKBannerView.m

@@ -38,7 +38,7 @@ - (void)createViewIfCan {
                                                     adSize:fbAdSize
                                         rootViewController:RCTPresentedViewController()];
 
-  adView.frame = CGRectMake(0, 20, adView.bounds.size.width, adView.bounds.size.height);
+  adView.frame = CGRectMake(0, 0, adView.bounds.size.width, adView.bounds.size.height);
   adView.delegate = self;
 
   [adView loadAd];