Bump pg_search from 2.0.1 to 2.3.0

Using pg_search 2.0.1 with Rails 5.2 results in deprecation warnings:

DEPRECATION WARNING: Dangerous query method (method whose arguments used
as raw SQL) called with non-attribute argument(s):
"pg_search_978c2f8941354cf552831b.rank DESC, \"tags\".\"id\" ASC".
Non-attribute arguments will be disallowed in Rails 6.0. This method
should not be called with user-provided values, such as request
parameters or model attributes. Known-safe values can be passed by
wrapping them in Arel.sql().

We're not upgrading to the latest pg_search because it only supports
ActiveRecord >= 5.2.

app/models/concerns/searchable.rb

@@ -2,7 +2,7 @@ module Searchable
   extend ActiveSupport::Concern
 
   included do
-    include PgSearch
+    include PgSearch::Model
     include SearchCache
 
     pg_search_scope :pg_search, ->(query) do