For some reason (probably some problem with one of the serializers) we
sometimes lack an id attribute for a promise that we get for a repo
relationship on build. Because of that doing `build.get('repo.id')` may
sometimes return undefined. A temporary workaround is to make sure that
we always can access the `repository_id` property.
One thing that is not standard here is a serializer for branch, which
uses @href as id. At this point branches don't have ids and ember-data
needs one, so using @href is the easiest way.
This commit adds adapters and serializers for v3, but also a fallback
serializer for v2, which allows to handle v2 and v3 payloads at the same
time. This is needed, because when we use v3 endpoint for one of the
models (in this case repo), we can also get embedded records of other
types (like branch or build).
