Fix mobile (safari) visual bug

The edges of the lightbox were getting cut off due to the
react-modal style settings. For some reason this didn't come
up on desktop browsers.

Author: fritz-c

src/react-image-lightbox.js

@@ -855,6 +855,10 @@ class ReactImageLightbox extends Component {
                 backgroundColor: 'transparent',
                 border:          'none',
                 borderRadius:    0,
+                top:             0,
+                left:            0,
+                right:           0,
+                bottom:          0,
             },
         };