Review `http:` strings in plugin and update to `https:` where possible

[benlk]

https://github.com/INN/pym-shortcode/blob/ee341bd307754ea1025a98ddcfe235165c04aca4/pym-shortcode.php#L8

This was reported by @claudiulodro in Automattic/newspack-plugin#276 (comment)