Added percentile ranks aggregation

Aggregation/PercentileRanksAggregationTest.php

+<?php
+
+/*
+ * This file is part of the ONGR package.
+ *
+ * (c) NFQ Technologies UAB <info@nfq.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace ONGR\ElasticsearchBundle\Tests\Unit\DSL\Aggregation;
+
+use ONGR\ElasticsearchBundle\DSL\Aggregation\PercentileRanksAggregation;
+
+class PercentileRanksAggregationTest extends \PHPUnit_Framework_TestCase
+{
+    /**
+     * Tests if exception is thrown.
+     *
+     * @expectedException \LogicException
+     */
+    public function testIfPercentileRanksAggregationThrowsAnException()
+    {
+        $agg = new PercentileRanksAggregation('foo');
+        $agg->toArray();
+    }
+}