As of time of writing, probably to 2.0.2 - https://github.com/Seldaek/monolog/releases/tag/2.0.2
https://github.com/Seldaek/monolog/releases/tag/2.0.1
https://github.com/Seldaek/monolog/releases/tag/2.0.0
There are some breaking changes, but not sure offhand if anything that affects us - https://github.com/Seldaek/monolog/blob/2.0.0/UPGRADE.md