LightBox srcSet property to support string and array

[lordoffreaks]

Steps to reproduce the behavior:
Using gatsby the srcSet property returned by graphql is a string not an array
Expected behavior:
work
Actual behavior:
Invalid prop images[0].srcSet of type string supplied to Lightbox, expected array.

The proposed fix would be to set
srcSet: PropTypes.oneOfType([PropTypes.string, PropTypes.array]),
in https://github.com/jossmac/react-images/blob/master/src/Lightbox.js#L378 as done in https://github.com/neptunian/react-photo-gallery/blob/master/src/Photo.js#L28

Thanks!