PHP 7.1 Compatibility

[silvioq]

Hi. I 'm using the library with PHP7.1

The warning "A non well formed numeric value encountered" is thrown here when I'm using margin-top with units (see http://php.net/manual/en/migration71.other-changes.php)

https://github.com/psliwa/PHPPdf/blob/7055c857a34003b8bf2af6200d5fad18360a67a6/lib/PHPPdf/Core/Node/Page.php#L421

$ ~/develop/composer.phar info | grep psliwa
psliwa/pdf-bundle                    1.0.3              This bundle integrates Symfony2 with PHPPdf library.
psliwa/php-pdf                       1.2.10             Pdf and graphic files generator library for PHP.

$ php -v
PHP 7.1.11-1+ubuntu14.04.1+deb.sury.org+1 (cli) (built: Oct 27 2017 14:28:45) ( NTS )
Copyright (c) 1997-2017 The PHP Group
Zend Engine v3.1.0, Copyright (c) 1998-2017 Zend Technologies
    with Zend OPcache v7.1.11-1+ubuntu14.04.1+deb.sury.org+1, Copyright (c) 1999-2017, by Zend Technologies

As workaround, I changed mm units for numeric value.