Update gds-sso

The version being used here is out of date and means we can't alter the
Gemfile at all.
Install Logstasher for log formatting
Enable and configure logstasher in production
Turn off lograge

Since we're using JSON logging now we don't need lograge.

Also remove it from the Gemfile.
Merge pull request #66 from alphagov/logstasher-format

Logstasher format