UX fix for remote signout

http://git.io/llA6Qw
govuk_content_models needed to be updated
to get this version of gds-sso through.
Fix usage of User.find_by_uid

the `find_by_*` methods are an ActiveRecord thing, and Mongoid doesn't
implement these.  This was only working because earlier versions of
gds-sso added a `find_by_uid` method to the User model.
Merge pull request #157 from alphagov/use-latest-gds-sso

UX fix for remote signout