Upgrade from 3.x to 4.0

Dependencies

Support for Symfony < 3.2 has been dropped. Support for Twig < 2.4.2 has been dropped. Support for PHP < 7.1 has been dropped.

API

Type hinting has been added to most classes.

`Greg0ire\Enum\AbstractEnum`

getEnumTypes may no longer return a string, it should always return an array now.