Add explicity void return type to Monolog\Logger

[nicolas-grekas]

As part of its migration to PHP 7.2, Symfony is starting to require more explicit annotations for return-type. Of course, Monolog cannot add real ones here, due to BC. These declarations would help us (and would make our CI green again as we now have a check to ensure the code and its deps are in good shape in this regard, see e.g. https://travis-ci.org/symfony/symfony/jobs/571836979)

Cannot be applied on branch 1.x since it has conflicting @return bool.

[nicolas-grekas]

Now with real return-types, all green.
The change doesn't affect neither the Symfony bridge not the bundle if I made no mistakes.

[Seldaek]

Thanks, let's hope this goes as planned ;)