Change plugins.svn linting to PHP 7.current please

As a follow up to and can we please switch the PHP linting for plugins.svn to 7.2 and keep it up-to-date with newer PHP branches as they’re deployed for

Although this is specifically for plugins.svn, the same should be applicable to all SVNs now, as any which require an older PHP version will be enforced through Travis and other tools.

Having the linter exclude newer syntax is becoming a limitation for plugin developers and with the work being done around plugins specifying minimum PHP versions, and increasing the WordPress minimum versions, is going to result in more plugins requiring PHP 7.1/7.2/etc.