Bump gds-sso to 9.3.0

Signon will start rejecting requests that don't include client_id in the
Near Future. Upgrading to 9.3.0 makes this app compatible before that
change happens.

This version of gds-sso has a dependency on multi_json 1.10.0, so bump
that too.
Merge pull request #176 from alphagov/bump-gds-sso

Bump gds-sso to 9.3.0