AtD: use HTTPS for requests to afterthedeadline.com when SSL is on

The initial proofread works fine and underlines words. But if you try to use Explain which makes a remote request Firefox blocks this by default without making an exception.
Here is the error:
```
Blocked loading mixed active content "http://service.afterthedeadline.com/info.slp?text=has+been+tested&tags=VBZ%2FVBN%2FVBN&engine=3&theme=tinymce&ver=358-24485"
```

If you just changed this to https://service.afterthedeadline.com/info.slp that should fix it.
Reported here:
* http://wordpress.org/support/topic/jetpack-proofread-explain-and-admin-ssl?replies=1

Original trac ticket: 
* https://plugins.trac.wordpress.org/ticket/1886