@JsonApiRelationship fails for other collection types than List

[Brutus5000]

Seems that only List is supported as of now but could be probably replaced with Collection instead:

https://github.com/cradlepoint/JsonAPIary/blob/master/src/main/java/com/cradlepoint/jsonapiary/serializers/JsonApiSerializer.java#L310-L329

Unfortunately I can't build and verify it under Windows (#13)