Update index.php in install folder

I have added a control version for php.
Users should kwon which php version is required to install PyroCMS before run composer install in order to avoid the last step problem.
If users try to install Pyro with a lower version the installation process stops to the 4th step without any message.
With this fix users already kwon wat they need
Merge pull request #2898 from ChristianGiupponi/patch-3

Sounds like a good idea to me.