Suppress "Delegating exists to arel is deprecated" warning

This commit suppresses the following Active Record warnings.

```console
DEPRECATION WARNING: Delegating exists to arel is deprecated and will be
removed in Rails 6.0. (called from pull_request_filter at
/Users/koic/src/github.com/24pullrequests/24pullrequests/app/models/aggregation_filter.rb:7)

DEPRECATION WARNING: Delegating with to arel is deprecated and will be
removed in Rails 6.0. (called from block (4 levels) in <top (required)>
at /Users/koic/src/github.com/24pullrequests/24pullrequests/spec/controllers/users_controller_spec.rb:16)
```

This patch is based on the following comment.
rails/rails#29619 (comment)

app/models/aggregation_filter.rb

@@ -4,6 +4,6 @@ class AggregationFilter < ApplicationRecord
   def self.pull_request_filter
     where("pull_requests.user_id = aggregation_filters.user_id")
     .where("pull_requests.title ILIKE aggregation_filters.title_pattern")
-    .exists.not
+    .arel.exists.not
   end
 end

spec/controllers/users_controller_spec.rb

@@ -13,7 +13,7 @@
         get :index
       end
 
-      it { expect(assigns(:users).with(User.order('pull_requests_count desc').page(0))).to be_truthy }
+      it { expect(assigns(:users).arel.with(User.order('pull_requests_count desc').page(0))).to be_truthy }
     end
 
     context 'as json' do