Update extension to use new version of SymfonyMockerContainer

[SymfonyMockerContainer](https://github.com/PolishSymfonyCommunity/SymfonyMockerContainer) needs merging and tagging [PR4](https://github.com/PolishSymfonyCommunity/SymfonyMockerContainer/pull/4) as version 2.0 before merging this PR.