No longer expose gravatar_email in docker.io api

Docker.io API has replaced the gravatar_email field with a
gravatar_url field instead.

Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Merge pull request #4938 from jlhawn/master

No longer expose gravatar_email in docker.io api