Can't install with SF4

[Perf]

When trying to install EntityMerger in Symfony 4 project it shows error, due to symfony/serializer of version ^4.0 is not supported.
Please update dependencies.

    "require": {
        ...
        "symfony/serializer": "^2.3|^3.0|^4.0",
        ...
    }

[Pierstoval]

I'm not sure this project should still be alive, actually. Merging two objects together has lots of problems, and much better & prettier solutions exist to fix this business need, as talked in #1 😄