Fix installation when omniauth-gds isn't installed

The gemfury repository was removed, which meant that if you didn't already have
the omniauth-gds gem installed, it couldn't be found. omniauth-gds 0.0.4 is
available in rubygems.org and gds-sso allows ~> 0.0.3, so upgrading fixed the
problem and removed the dependency on Gemfury.