Bump API adapters to 4.9.0 for relative URLs.
Instantiate content API adapter with a site root.

This makes the API adapter transform absolute `web_url` values which
point to the website root into relative URLs, so we can safely display
them on staging and private frontend instances without accidentally
linking through to the live site.
Update browse page tests to expect relative URLs.
Merge pull request #232 from alphagov/relative-api-urls

Relative API URLs