tests/gravatar: another https:// fix

ffbd258b00ae8736af1b8ad6bbe7c3dd2fd8caba was supposed to fix the
Gravatar test, but there were actually two places where http:// was
changed to https://, and that commit only corrected for one.  Correct
the other one here.

Signed-off-by: Daniel Harding <dharding@living180.net>