Use `plural` instead of `pluralize` with ModelName instance in `graph_data`

Fixes Rails 4, rails_admin 0.5.0 'undefined method `pluralize' for #<ActiveModel::Name:0x007f93a0175990>'
Merge pull request #1 from nthj/patch-1

Use `plural` instead of `pluralize` with ModelName instance in `graph_data`