Bump to latest gds-sso

Newest version sets an appropriate response content-type, which will
stop slimmer from attempting to process the empty responses on reauth
calls
Override gds-sso authentication failure template

This page is rarely seen, but because it lacks the expected wrapper
div in the gds-sso gem, slimmer throws a wobbly when it tries to process
it. By overriding the template here in whitehall, we can ensure the
appropriate markup is present to keep slimmer happy.
Merge pull request #1505 from alphagov/bump-gds-sso

Fix gds-sso/slimmer-related issues