Bump gds-sso to 9.4.0, pick up User specs

This reduces the risk of refactorings breaking the GDS SSO integration
when the `User` class is refactored or `gds-sso` is bumped.
Merge pull request #207 from alphagov/add-gds-sso-lint-tests

Bump gds-sso to 9.4.0, pick up User specs