There was an earlier version of this thread, now closed, but the solution proposed there no longer works with Wordpress 4.0
https://wordpress.org/support/topic/stripping-inline-width-style-for-wp_caption-on-images?replies=4
I also found additional solutions here:
http://labs.travelgrove.com/news/how-to-fix-wordpress-3-7-image-caption-bug/
But, again, none of these seemed to help with Wordpress 4.0.
I'm posting this new thread in the hope that someone can offer a solution.