Symfony 5 support

The bundle does not appear to support the newly released Symfony 5. When attempting to upgrade an installation with the software installed, I get the following error:

```
backup-manager/symfony 2.2.0 requires symfony/console ^2.7 || ^3.1 || ^4.0 -> satisfiable by symfony/console[v4.3.8] but these conflict with your requirements or minimum-stability.
```