Update gds-sso gem

We need a newer version to get the organisation_slug back from signon
when doing user auth.

Basic auth support has been removed in the latest version.
User -> Organisation relationship keys by slug

Also includes a pending migration to drop organisation_id.
Remove ability to edit user organisation & email

Also tighten up the mass assignment protection for User and fix a few
tests that were using User.create and falling foul of the new changes.
Merge pull request #1099 from alphagov/get-user-organisation-from-signon

Get user organisation from signon

[Delivers https://www.pivotaltracker.com/story/show/60396766]