add support for PHP8.0 

Fixed for PHP 8 Compatibility 

Merge pull request #338 from mitelg/rename-class Rename MatrixStatsAggregation.php to MaxAggregation.php

Rename MatrixStatsAggregation.php to MaxAggregation.php 

support serializer v5 and upgrade php minimum requirement to the 7.2 

add elasticsearch/elasticsearch to the composer file 

update travis 

rollback support for v3 and v4 serializer 

upgrade elasticsearch version in the travis to the 7.5.2 

update to support the symfony serializer 5.0