Fix NameError from nulldb (#34)

nulldb is only loaded in the tests, so using its names in library code raised an exception:

```
NameError: uninitialized constant ActiveRecord::ConnectionAdapters::NullDBAdapter
```

We can instead key off the method that nulldb doesn't implement to fix the same issue as #32.

lib/acts_as_scrubbable/ar_class_processor.rb

@@ -28,7 +28,7 @@ def process(num_of_batches)
       end
 
       ActsAsScrubbable.logger.info Term::ANSIColor.blue("#{scrubbed_count} #{ar_class} objects scrubbed")
-      ActiveRecord::Base.connection.verify! unless ActiveRecord::Base.connection.is_a?(ActiveRecord::ConnectionAdapters::NullDBAdapter)
+      ActiveRecord::Base.connection.verify! if ActiveRecord::Base.connection.respond_to?(:reconnect)
 
       ActsAsScrubbable.logger.info Term::ANSIColor.white("Scrub Complete!")
     end

lib/acts_as_scrubbable/task_runner.rb

@@ -55,7 +55,7 @@ def scrub(num_of_batches: nil, skip_before_hooks: false, skip_after_hooks: false
       Parallel.each(ar_classes) do |ar_class|
         ActsAsScrubbable::ArClassProcessor.new(ar_class).process(num_of_batches)
       end
-      ActiveRecord::Base.connection.verify! unless ActiveRecord::Base.connection.is_a?(ActiveRecord::ConnectionAdapters::NullDBAdapter)
+      ActiveRecord::Base.connection.verify! if ActiveRecord::Base.connection.respond_to?(:reconnect)
 
       after_hooks unless skip_after_hooks
     end

lib/acts_as_scrubbable/version.rb

@@ -1,3 +1,3 @@
 module ActsAsScrubbable
-  VERSION = '2.1.2'
+  VERSION = '2.1.3'
 end