Update signon integration to fix bearer token issue

See https://github.com/alphagov/signonotron2/pull/245 for more details.

This updates the sso callback auth implementation, as well as bumping
omniauth-gds to pick up the fix for both auth flows (regular oauth, and
bearer token).
Merge pull request #23 from alphagov/fix_bearer_token

Update signon integration to fix bearer token issue