Update test fixtures

Now we're bundling some API specs as Ruby Gems, consider removing the fixtures from Git and adding the Gems as dependencies, e.g.

```ruby
source 'https://rubygems.pkg.github.com/epimorphics' do
  gem 'fsa-cbd-api'
  gem 'fsa-regulated-products-api'
end
```

Note that these Gems are currently private to the organisation.