Fix conditional notifiers

In trying to use conditional notifiers, I noticed it was blowing up with an error similar to the following:

```
 WARN batsir: Caught NoMethodError: undefined method `new' for #<MyModule::Notifiers::TrueNotifier:0x007fdcee054920>
	/Users/dczarnecki/projects/batsir/lib/batsir/notifiers/conditional_notifier.rb:23:in `block in execute'
	/Users/dczarnecki/projects/batsir/lib/batsir/notifiers/conditional_notifier.rb:18:in `each'
	/Users/dczarnecki/projects/batsir/lib/batsir/notifiers/conditional_notifier.rb:18:in `execute'
	/Users/dczarnecki/projects/batsir/lib/batsir/notifiers/notifier.rb:24:in `notify'
	/Users/dczarnecki/projects/batsir/lib/batsir/stage_worker.rb:24:in `block in execute'
	/Users/dczarnecki/projects/batsir/lib/batsir/stage_worker.rb:23:in `each'
	/Users/dczarnecki/projects/batsir/lib/batsir/stage_worker.rb:23:in `execute'
```

I printed out the generated code and noticed the following:

```
def self.initialize_filter_queue
  @filter_queue = Batsir::FilterQueue.new
  @filter_queue.add MyModule::Filters::StatisticsFilter.new({})
  @filter_queue.add MyModule::Filters::NoopFilter.new({})
  conditional_notifier = Batsir::Notifiers::ConditionalNotifier.new
  condition = ->(message){2 > 1}
  conditional_notifier.add_notifier condition, MyModule::Notifiers::TrueNotifier.new({})
  conditional_notifier.add_transformer Batsir::Transformers::JSONOutputTransformer.new
  @filter_queue.add_notifier conditional_notifier
end
```

So the generated code is already instantiating an instance of the notifier class to the `add_notifier` method. 

* The `ConditionalNotifierDeclaration#compile` method instantiates
  an instance of the notifier, so do not try and `new` the
  instance again.
* Update specs to reflect what the `ConditionalNotifierDeclaration#compile`
  method is doing behind the scenes.