Bump gds-sso to 9.3.0

This is to fix an issue with transfer ot bearer tokens. See
https://github.com/alphagov/signonotron2/pull/245 for details.

Bumped plek to get development mode default values.
Merge pull request #191 from alphagov/bump_gds-sso

Bump gds-sso to 9.3.0