Use HTTPS for speedtest.net URLs

HTTP is insecure – any attacker with a privileged network position
could intercept the request and possibly drop/rewrite it. HTTPS
makes it much harder and much more expensive for anyone to do so.

Some more reasons, taken from
<https://konklone.com/post/switch-to-https-now-for-free>:

* SSL’s not perfect, but we need to make surveillance as expensive
  as possible.
* For privacy not to be suspicious, privacy should be on by
  default.