Compatibility with Symfony 4

[bartosz-maciaszek]

Hi, I wanted to use the library with Symfony 4, but it turned out it supports ~2.3 and ~3.0 only. This is a quick fix to make it working with ~4.0.

[othillo]

Nice, we do have to update .travis.yml though to actually run the tests with Symfony 4.

[bartosz-maciaszek]

That won't be as simple as adding - SYMFONY_VERSION="4.0.*" as Sf4 requires at least PHP 7.1.3. Any idea how to do it neatly?

[othillo]

I will pick it up in #16

[othillo]

thank you, @bartosz-maciaszek !