Remove version number from composer require command

This avoids having to update README with each minor/major release.

README.md

@@ -38,7 +38,7 @@ recommend using the [app skeleton](https://github.com/cakephp/app) as
 a starting point. For existing applications you can run the following:
 
 ``` bash
-$ composer require cakephp/cakephp:"~3.6"
+$ composer require cakephp/cakephp
 ```
 
 ## Running Tests