added an abstract pipeline agg class

src/Aggregation/Pipeline/AbstractPipelineAggregation.php

+<?php
+
+namespace ONGR\ElasticsearchDSL\Aggregation\Pipeline;
+
+use ONGR\ElasticsearchDSL\Aggregation\AbstractAggregation;
+
+abstract class AbstractPipelineAggregation extends AbstractAggregation
+{
+
+}
\ No newline at end of file