test/gravatar_test: check for https:// URL

d53ec734cb9a183e9b50ffe86dabe631834651b5 broke the Gravatar test
because it was checking for a http:// URL from url_for_email(), but that
commit changed the generated URL to use https://.  Update the test to
check for a https:// URL.

Signed-off-by: Daniel Harding <dharding@living180.net>
Merge pull request #397 from living180/fix_tests

tests: fix Gravatar test

Signed-off-by: David Aguilar <davvid@gmail.com>