Make predict_php 7.x requirement clearer

The PHP predictions processor requires PHP 7.x to work because it is what php-ml library requires. We need to make this requirement clearer. We can not do it at plugin level because people can just use the python predictions processor or any other plugin they write, but we could show a warning message in the admin settings page if the server php version is lower than 7.0 and the PHP predictions processor is selected (it is the default value)