Remove root and instance_options for active model serializers 1.0.x compatability

[nikolai-b]

Very sorry this has no specs 😞
With the new https://github.com/rails-api/active_model_serializers version was failing with

       Diff: 

       @@ -1,6 +1,6 @@
       -"instance_options" => {},
       +"instance_options" => {"url_options"=>{"host"=>"test.host", "port"=>nil, "protocol"=>"http://", "_recall"=>{"version"=>"1", "controller"=>"...", "action"=>"..."}, "version"=>"1"}, "root"=>false, "singular"=>true},
        "object" => {"id"=>1, ...},
       -"root" => nil,
       +"root" => false,
        "scope" => nil,
        "url" => nil,

There is probably a more elegant way of doing this.