content_api_has_countries should accept a slug and hash

There are cases where we'll provide an updated_at value as well as
just the slug and name for a country.
Provide a way of parsing 'updated_at' values to Time values

Rather than duplicating a lot of this effort in every app, provide a
way of doing this at the API adapters level.
Merge pull request #45 from alphagov/update-has-countries

Update content_api_has_countries helper and pre-process times