CONTRIBUTING: minor tweak

Follow up to #809 and #796

Author: jrfnl

.github/CONTRIBUTING.md

@@ -44,11 +44,9 @@ gh repo clone Automattic/VIP-Coding-Standards vipcs
 Now `cd vipcs` and run:
 
 ```sh
-composer install --ignore-platform-req=php+
+composer install
 ```
 
-The platform requirements for higher versions of PHP are ignored so that the correct version of PHPUnit (7.x needed by PHPCS) is installed.
-
 You can now run:
 
 ```