Fix PHP version requirement

[kyborek]

README.md says

Requirements
This library package requires PHP 5.4 or later and Plesk 12.0 or above.

composer.json says

"require": {
        "php": ">=5.3.0"
    }

So which one is it, does it require PHP >=5.3 or >=5.4? Either way, the version numbers should be updated to match.

[kyborek]

Due to short array syntax the actual required php version is >=5.4.0

The composer.json file should be updated to reflect this

[kyborek]

At last - the highest version supporting PHP 5.3 seems to be tag 0.4.1