protected visibility is discouraged (#1687)

Coming from other languages, it's tempting to use `protected` in Ruby.
However this keyword has different nuanced in Ruby and is generally
discouraged. It should only be used for methods required for comparators
and operators. None of the methods marked as `protected` here fall into
this category, so I think they should be marked `private` instead.

lib/administrate/field/associative.rb

@@ -11,7 +11,7 @@ def associated_class
         associated_class_name.constantize
       end
 
-      protected
+      private
 
       def associated_dashboard
         "#{associated_class_name}Dashboard".constantize.new

lib/administrate/field/polymorphic.rb

@@ -23,7 +23,7 @@ def selected_global_id
         data ? data.to_global_id : nil
       end
 
-      protected
+      private
 
       def associated_dashboard(klass = data.class)
         "#{klass.name}Dashboard".constantize.new

lib/administrate/page/base.rb

@@ -23,7 +23,7 @@ def item_includes
         dashboard.try(:item_includes) || []
       end
 
-      protected
+      private
 
       def attribute_field(dashboard, resource, attribute_name, page)
         value = get_attribute_value(resource, attribute_name)

lib/administrate/page/form.rb

@@ -20,7 +20,7 @@ def page_title
         dashboard.display_resource(resource)
       end
 
-      protected
+      private
 
       attr_reader :dashboard
     end