Load Gravatar images always through HTTPS

This patch will force to load Gravatar images always through HTTPS which
will fix the problem with mixed content when accessing the page through
HTTPS.

The problem is that if an HTTPS page includes HTTP content, the HTTP
portion can be read or modified by attackers, even though the main page
is served over HTTPS.
Merge pull request #194 from jtyr/gravatar_https

Load Gravatar images always through HTTPS