fix: setting initial tab behaviour for iOS RN 0.59.x (#762)

Fix an issue where the initial tab is ignored on iOS and React Native 0.59.x when the initial tab is not the first tab in the set.

This uses the newly introduced scrollToOverflowEnabled prop: facebook/react-native@e3ac329

It seems counterintuitive to have this along with `overScrollMode="never"` but overScrollMode really controls the ability to scroll past the ends of a ScrollView, while scrollToOverflowEnabled allows us to manually `scrollTo()` an arbitary position out of bounds of the ScrollView.

I'm unusual of the underlying change in RN0.59.x that caused this, but it seems that the ScrollView hasn't completed layout when the `_setInitialPage()` is called, so it's unable to scroll to a non-zero x value.

src/PagerScroll.js

@@ -150,6 +150,7 @@ export default class PagerScroll<T: *> extends React.Component<
         keyboardDismissMode="on-drag"
         keyboardShouldPersistTaps="always"
         overScrollMode="never"
+        scrollToOverflowEnabled
         scrollEnabled={this.props.swipeEnabled}
         automaticallyAdjustContentInsets={false}
         bounces={false}