Use max-width instead of width for class="flickr-credit"

To make sure that the caption under an image isn't wider than the image itself and divides over more lines if necessary, a width is applied that equals the image width. That causes too much white space between the text and the image in case we have a portrait orientated image.

This can be easily solved by applying "max-width" instead of "width".

The max-width property is supported in all major browsers.
Note: The value "inherit" is supported in IE8 if a !DOCTYPE is specified. IE9 supports "inherit".
Note: IE6 and earlier versions do not support the max-width property.