Apply Style/ClassCheck rubocop rule

We were already using `is_a?` almost everywhere.

.rubocop.yml

@@ -373,6 +373,9 @@ Style/AndOr:
 Style/BlockDelimiters:
   Enabled: true
 
+Style/ClassCheck:
+  Enabled: true
+
 Style/Not:
   Enabled: true
 

app/controllers/admin/budget_investments_controller.rb

@@ -84,7 +84,7 @@ class Admin::BudgetInvestmentsController < Admin::BaseController
 
     def load_investments
       @investments = Budget::Investment.scoped_filter(params, @current_filter).order_filter(params)
-      @investments = Kaminari.paginate_array(@investments) if @investments.kind_of?(Array)
+      @investments = Kaminari.paginate_array(@investments) if @investments.is_a?(Array)
       @investments = @investments.page(params[:page]) unless request.format.csv?
     end