Avatar wrapper view should not be 'flex: 1'

[prithsharma]

https://github.com/react-native-material-design/react-native-material-design/blob/master/lib/Avatar.js#L56
Because the avatar wrapper view is flexible, it can't be aligned at the place where this component is used.

This is also why the demo app had to use a paddingTop instead of justifyContent: center for making the avatar look like it's in the center.

Even if we keep the wrapper view flexible, at least it should align and justify the children.