Upgrade from 1.5 to 1.6

Aggregation builder

Fixes a BC break when converting expression objects in the Doctrine\MongoDB\Aggregation\Expr class.
Adds extension points to customize expression conversion in the following stages:
- $bucket
- $bucketAuto
- $graphLookup
- $replaceRoot

Support for MongoDB server versions below 3.0 has been dropped. These are no longer supported by MongoDB. We recommend you upgrade to a recent version
The 1.5 version of the legacy driver is no longer supported. This library now requires at least version 1.6.7 of the legacy driver.
PHP 7.2 is also supported using mongo-php-adapter