backgroundSource propTypes is not object

[someok]

backgroundSource propTypes should be:

PropTypes.oneOfType([
      PropTypes.shape({
        uri: PropTypes.string,
      }),
      // Opaque type returned by require('./image.jpg')
      PropTypes.number,
    ]),

[nick-merrill]

@someok Why don't you submit a pull request so we can get this merged in?

[preco21]

PR created: #28