Symfony 4 support

v2: symfony 4 support

* Update composer requirements: php 7.1, symfony 4.0, phpunit 7.1

* Create constraints form and entity readers

* Remove constraints builders, use factories instead

* Move unit tests into Tests/

* Rename ParsleyAssert\Type => ParsleyAssert\Email

* Move classes

* Rework bundle configuration

* Better support for Type constraints

* Avoid skipped tests

* Update ci config