Infer plurality from rel instead of title

We currently use the link `title` to infer plurality of the response
object(s) for the purposes of response validation:

https://github.com/heroku/committee/blob/master/lib/committee/response_validator.rb#L11

As suggested by @cyberdelia it may be preferable to instead use
the `rel` looking for `"instances"`.

@brandur if you're open to that I can submit a patch.

Ref heroku/direwolf#184.