Update existing Attachable assets on update

When an asset already exists (and the 'update_existing' option has
been provided), update the asset using the API client rather than
creating a new asset each time.
Remove use of `any_instance` in Attachable tests
Update gds-api-adapters version

New AssetManager#update_asset method required.
Set Attachable update option on Attachments
Merge pull request #156 from alphagov/attachable-update-assets

Attachable update assets