Modified composer json for support symfony 4.2

[CedrickOka]

This pull request should be able to use the project with Symfony 4.2

[coveralls]

Coverage increased (+0.08%) to 99.692% when pulling 16f3d06 on CedrickOka:master into 21058a9 on jkphl:master.

[coveralls]

Coverage increased (+0.08%) to 99.692% when pulling 16f3d06 on CedrickOka:master into 21058a9 on jkphl:master.

[CedrickOka]

Hi @jkphl
I would like to use this library in my project symfony 4 is what you can annalyser my pull request that should allow me to do it.

[jkphl]

@CedrickOka Thanks for your suggestion — this should be done now as of release 3.0.0.

On another note: While I appreciate your contribution, please try to not push by "reminding" me that there's an open PR as this doesn't help at all with my overall workload. In this concrete situation: Allowing Symfony 4.2 required bumping the PHP version constraint, which in turn had a lot of impact on other dependencies as well, so it was far more involved than to just modify the composer.json ...

[CedrickOka]

@jkphl Thanks