Suppress Rails 6.0's warning

Follow up of rails/rails#35093.

This PR suppresses the following warning when using Rails 6.0.

```console
% path/to/buoys
% bundle exec rake

(snip)

DEPRECATION WARNING: ActionView::Base instances should be constructed
with a lookup context, assignments, and a controller. (called from new
at /Users/koic/src/github.com/muryoimpl/buoys/test/buoys_buoy_test.rb:7)
```

test/buoys_buoy_test.rb

@@ -4,7 +4,11 @@
 
 class BuoysBuoyTest < ActiveSupport::TestCase
   setup do
-    @view_context = ActionView::Base.new
+    if ActionView::Base.respond_to?(:empty)
+      @view_context = ActionView::Base.empty
+    else
+      @view_context = ActionView::Base.new # Rails 5.2 or lower
+    end
     Buoys::Loader.load_buoys_files
   end