Update Plek to 1.0

This change is required to get Whitehall up to speed with the new
preview environment.

A few breaking changes have been introduced:

- `GOVUK_ASSET_ROOT` is now required to be set
- `GOVUK_APP_DOMAIN` is now required to be set

These are both set for Jenkins and are automatically set if you use the
vagrant-based development environment and Bowler/Foreman.