Add not_sharded back in with a deprecation warning.

We are method chaining not_sharded in ZendeskDatabaseSupport and adding default
reading from the slave for unsharded models. To help the transition we'll keep
the not_sharded method around until we can remove it from the dependents.

lib/active_record_shards/model.rb

@@ -2,6 +2,13 @@
 
 module ActiveRecordShards
   module Model
+    def not_sharded
+      ActiveSupport::Deprecation.warn("Calling not_sharded is deprecated. "\
+                                      "Please ensure to still read from the "\
+                                      "account db slave after removing the "\
+                                      "call.")
+    end
+
     def is_sharded? # rubocop:disable Naming/PredicateName
       self <= ActiveRecordShards::ShardedModel
     end