As google (and the internet) is moving towards https:// everywhere, I thought I check a few of the most common plugins I use for links using http://. Here are the results for this plugin:

https://github.com/Yoast/wordpress-seo/search?utf8=%E2%9C%93&q=http%3A%2F%2F

Based on these results, I suggest doing a few find/replace to encourage https:// whenever possible. Here are just a few domains that can be searched and updated:

http://wordpress.org
http://make.wordpress.org
http://example.org
http://www.gnu.org
http://www.w3.org
http://www.google.com
http://translate.yoast.com
http://php.net
http://schema.org
http://en.wikipedia.org
http://www.facebook.com

http://kb.yoast.com/ --> Doesn't yet support HTTPS