JSON-API: Don't include linked section if associations are empty
Fixes rbx gems bundling on TravisCI
Merge pull request #711 from rails-api/fixes-rbx-on-travis

Fixes rbx gems bundling on TravisCI
Merge pull request #710 from ride/fix-empty-linked

JSON-API: Don't include linked section if associations are empty
Include 'linked' member for json-api collections

The options passed to the render are partitioned into adapter options
and serializer options. 'include' and 'root' are sent to the adapter,
not sure what options would go directly to serializer, but leaving this
in until I understand that better.