attr_accessible only called if defined in GDS::SSO::User

attr_accessible and attr_protected have been removed in Rails 4
favoring using strong paramaters instead. This change prevents large,
and unhelpful explosions when a rails 4 model includes GDS::SSO::User.
Fix find_for_gds_oauth in Rails 4

Fixes #33 where update_attributes does not take options in Rails 4.
Also uses Gem::Version for comparing gem versions, seems to be the way
most gems do this. see https://github.com/jnicklas/capybara/commit/78e3a488291fcc28e8293a25f3d5f1e95741d362
Reverse conditions to be clearer
Merge pull request #32 from alphagov/fix-user-attr-accessible

attr_accessible only called if defined in GDS::SSO::User