All deprecated code has been removed. Support for Symfony 2 and anything lower then PHP 5.6 was dropped. Official support for HHVM is also dropped. You need at least Symfony 3.3 and PHP 5.6 (or PHP 7.0).
Upgrading
The constraints have been moved to a separate library, see the upgrade instructions for full details.
To ease transitioning your application to v2.0, first update to v1.7. v1.7 already provides the validation constraints while still allowing the old usage.
After this you can update your code, The Symfony web-development toolbar should inform where you need to update your code to use the new constraints.
Once all deprecations are resolved you can update the application's composer.json file to use v2.0 of this bundle.