Remove maximum version for symfony/config and symfony/dependency-injection

I don't think there's any reason to limit these dependencies to <2.3.0. I've tested with 2.3.x and everything seems to work fine. I see no reason why 2.4 and higher wouldn't work as well.
Merge pull request #14 from wildlyinaccurate/patch-1

Remove maximum version for symfony/config and symfony/dependency-injection