1.6.0 - #18

Fix typo

* Test against Ruby 2.6

* Enforce bundler 1.17.3

* Allow relationship links to be declared as object method

* Relationship links note added to readme

* The link was already there but I skipped over it on my first read.
This update makes the fact more prominent.
* I was testing to see if we wanted to move from AM Serializers to
fast_jsonapi but our API is not written according to the JSON:API spec
so, after converting one serializer over, I learned that this would not work for me.
* This update might save someone in my position the ~30 mins or so it
takes to bundle and write a serializer in the future. :)

Fix typo

* Test against Ruby 2.6

* Enforce bundler 1.17.3

…`includes` (#10)

* Add params to set_id block arguments

Pull request #331 added a block to the ObjectSerializer.set_id class
method, which allows passing a block to the set_id method. Currently
this block takes only one argument `record`:

```
set_id do |record|
  "#{record.name.downcase}-#{record.id}"
end
```

This PR adds another argument `params` to the block:

```
set id do |record, params|
  params[:admin] ?  record.id : "#{record.name.downcase}-#{record.id}"
end
```

This customization can be useful in situation where we serve different
clients that may need different IDs. One nice side effect is also that
the `set_id` method has the same method signature as the `attribute`
method.

* Update the README