Fix gravatar_id to use email.

This is a temporary fix for travis-ci/travis-ci#2837, until the
sync is fixed.
Merge branch 'master' into mm-fix-gravatar-url
Auto-merged master into mm-fix-gravatar-url on deployment.
Merge pull request #143 from travis-ci/mm-fix-gravatar-url

Temporary fix to use Gravatar URL