Added checking symfony requirements before showing success message

I've added checking symfony requirements (as per #9)
If any of requirements is not met installer will show error message and list missing requirements. Hovewer, it will not show recommendations as app/check.php does that already and I think it's not needed here?
Probably error message can be better but I think it's a good start.

This PHPdoc block seems to be lacking one of the arguments of the method and the explanations of the returned value and the arguments (maybe it's not necessary because they are self-explanatory?)