remove dependency to symfony/symfony

[Metabor]

Libraries should not depend on metapackage symfony/symfony, but on the Symfony components they use.
See https://symfony.com/doc/current/setup/flex.html#upgrading-existing-applications-to-flex

[Metabor]

Also needed to switch to PHPUnit as a dev dependency to prevent this error:
https://stackoverflow.com/questions/49833939/php-fatal-error-parserparsefile-symfony-3-4-travis-ci

[Metabor]

Tests are fixed now. With the changes above this bundle can be used with symfony flex (and maybe it is also Symfony 4 compatible now, but I didn't checked it).

[PastisD]

@Metabor I can confirm that's working on Symfony 4 :)
Just had to change the compatibility in composer.yml (PastisD@6b46d87 We can even let the ~2.6 dependency)

Thx for the work :)