v2.0.0

Breaking

• Remove Express cookie utilities. These should be ported to s3o-middleware.
• Update dependencies. Breaks node 6 support due to ft-poller change.
• Return a factory function from authenticate.js.

See https://github.com/Financial-Times/s3o-middleware-utils/blob/master/MIGRATION.md#migrating-from-v1-to-v2 for a migration guide.

Other changes

• Add tests without stubs for validate function which create an RSA key pair.