Use "secure.gravatar.com" when the scheme is https.

According to the instructions from Gravatar...

http://en.gravatar.com/site/implement/images/

...the URL host should be "secure.gravatar.com"
instead of "www.gravatar.com".

The "www" address does (usually) work with https,
but I have noticed occasionally that it does not
when the "secure" address does.  Probably because
it requires some extra redirection or DNS settings
by Gravatar that aren't always working.