Fix another SF 4.2 deprecation

[xkobal]

A tree builder without a root node is deprecated since Symfony 4.2 and will not be supported anymore in 5.0.

[franmomu]

This bundle supports symfony/framework-bundle": "^3.3 || ^4.0 so the fix should be something like in doctrine/DoctrineBundle#853, that's the reason why https://travis-ci.org/geocoder-php/BazingaGeocoderBundle/jobs/466503002 fails

[xkobal]

@franmomu I have fixed my pull request as asked. it seems to work this time.

[Nyholm]

It looks like there is one more deprecation. Could you check that?

[xkobal]

The deprecation come from the PhpUnit version used on the test: 5.7.27.
On my computer, when I composer install (without flags) I have the 6.5.13 and I have no deprecation.

So I don't know how to upgrade the phpunit version. Seems to come from the composer flag passed in travis.yml (--prefer-stable --prefer-lowest --prefer-dist).

[Nyholm]

Thank you