Don't include attr_accessible in test model

This already gets set up when GDS::SSO::User is included.
Use Rails-4 compatible routing

match is deprecated in favour of get.
Avoid using class name that clashes

The code that is used to test the gem in the context of a rails app
already defines an ActiveRecord model called User. If we also define it
here, we prevent the other model being autoloaded in the other tests.
Use wwtd gem for multi-rails testing

wwtd allows you to run tets against multiple sets of gems as well as
different versions of ruby. It uses the same configuration as Travis CI,
hence the presence of a .travis.yml file and the gemfiles folder.