Bump gds-sso version, and requirements

I've rebuilt the Gemfile.lock, but needed to stop us moving to rspec
version 3.0 because of a bug in Cucumber
(https://github.com/cucumber/cucumber/pull/666) which has been fixed
upstream but doesn't seem to have been released yet.
Merge pull request #15 from alphagov/bump-gds-sso

Bump gds-sso version, and requirements