minor #625 Update php version requirement in readme (ScullWM)

This PR was merged into the master branch.

Discussion
----------

Update php version requirement in readme

Hi

Seems that symfony-demo in no longer available for php 5.5.9.
Composer.json define the minimum requirement to 7.1.3.

Readme update

Commits
-------

1ad02e4 Update php version requirement in readme

Author: javiereguiluz

README.md

@@ -9,7 +9,7 @@ to develop Symfony applications following the recommended best practices.
 Requirements
 ------------
 
-  * PHP 5.5.9 or higher;
+  * PHP 7.1.3 or higher;
   * PDO-SQLite PHP extension enabled;
   * and the [usual Symfony application requirements](https://symfony.com/doc/current/reference/requirements.html).