Monolog: only require basic monolog instance

[glaubinix]

This allows usage of the library without necessarily using the symfony/monolog-bundle/symfony/monolog-bridge. Like for instance in silex apps.

I left the dependency in the composer file the library still requires monolog to be available via symfony DI but this way the logger instance does not have to be the bridge instance but can be any Monolog instance.

[scrutinizer-notifier]

The inspection completed: No new issues

[k-k]

Thanks @glaubinix !