Deprecate/remove calls to _wp_upload_dir_baseurl()

[joemcgill]

WordPress core is looking at modifying and/or removing _wp_upload_dir_baseurl(). We need to remove this from our plugin, or at least make sure it's wrapped in a function_exists() check before it's called to make sure users with this plugin installed don't get fatal errors when upgrading to WP 4.5.

See: https://wordpress.slack.com/archives/core-images/p1457578517000379

This may get solved as a result of #269 if that ticket expands to include 4.5 changes.