Only strip the first slash in the publication slug

This breaks slugs containing slashes (eg. travel-
advice/portugal). I looked at this with
@jamiecobbett and I don't believe the gsub was
intentional, as there would be no other slashes to
remove from the path.
Add a RootController test for done/* pages.
Don't assume artefact has attributes methods

This is more consistent with PublicationPresenter
behaviour for looking up artefact attributes, and
by doing this, we can stub the api adapter in
tests to return a hash where necessary.
Bump gds-api-adapters to 4.2.0
Add travel-advice as a supported format
Fix test to check an "unfriendly" URL parameter

This test no longer fails due to the additional
URL escaping added in gds-api-adapters 4.2.0

I still think it's a good idea to have this test
to make sure we don't get any other unexpected 
behaviour in the app - so here the request is 
stubbed to return a 404.
Display a travel advice edition from the content API
Don't display part numbers for country guide navigation
Show the country name, not the artefact title

The artefact title appends the words 'travel advice'
Use the guide icon for travel advice editions
Change fetch method to return both artefact and publication
Add controller test for expiry headers
Move set_slimmer_artefact_headers into ApplicationController

Move this method out so that other controllers can
use it - eg for travel advice
Set the artefact slimmer headers for travel advice
Use the country parameter instead of params[:slug in fetch method
Make explicit the intention of PublicationPresenter's slug method
Make that slug regex a bit clearer :)
Change 'slug' to 'country_slug' in the travel advice controller

Makes a bit more sense, as country_slug is not the
entire slug, as we prepend it with 'travel-advice/'
Merge remote branch 'origin/travel-advice'