Automatic version detection

[uuf6429]

This pull request adds a new ruleset, PhpAuto, that picks the right ruleset based on the minimum required PHP version defined in composer.json of the current working directory.

New dependencies added:

1. ext-json (backword compatible, since it was already required by php-cs-fixer)
2. composer/semver (supported since PHP 5.3, so it should also be fine)

Additional Changes:

• project itself now uses the new ruleset (I figured it would be less maintenance + eating your own dog food)
• updated the readme file with the same reasoning; most people would want to automate this

Design Notes:

• at first I was going for a PHP version -> ruleset class (e.g. [53 => Php53::class]) map but then I thought it would be less work to just add an interface and build the class name dynamically