Fix "skip", "next" and "done" button positions on iOS

[fortinmike]

Fixes #111. Not tested on Android, so please do before merging.

[pedrofmlopes]

Hello, all you have to do to fix that issue is go to AppIntro.js file, inside react-native-app-intro and change flex inside styles.btnContainer to 1:

....
btnContainer: {
flex: 1,
justifyContent: 'center',
alignItems: 'center',
height: 50
},
...

Hope it helps

[arturowczarek]

You can also apply the same style adding custom style:

<AppIntro customStyles={{ btnContainer: {flex: 1} }}>
...
</AppIntro>```

[ChenLi0830]

Works, thanks @fortinmike & @arturowczarek !