Soundcloud Shortcode: update player url to be protocol agnostic

Steps to reproduce:

1. On a site using only HTTPS, activate Jetpack and the Shortcodes module
2. Embed a Soundcloud player in one of your posts, like so:
```
[soundcloud url="https://api.soundcloud.com/tracks/119453276" width="100%" height="166" iframe="true" /]
```
3. Publish
4. Open the post: you'll receive a mixed-content warning because the SoundCloud player is served via HTTP on a page using only HTTPS

Initial reported here:
* https://plugins.trac.wordpress.org/ticket/2038
* http://wordpress.org/support/topic/shortcode-plugin-mixed-content-warning-on-pages-served-over-tls?replies=1