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.

Does it really make sense to not show the next steps only because some of the requirements couldn't be satisfied? In my opinion, it's more user friendly to simply show the message below additionally to the success message.