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.

Was also necessary to add an additional field to the User model because
gds-sso hasn't been updated for a while.

Removed gemfury source line from Gemfile as it's redundant.
Merge pull request #29 from alphagov/bump_gds-sso

Bump gds-sso to 9.3.0
show sync exception
Merge pull request #30 from bitzesty/sync-exceptions

Store backtrace/SQL queries of failed syncs in the DB