Using lineno-start on code-block directive throws error

[brotkrueml]

Using this code block:

..  code-block:: ruby
    :lineno-start: 10

    Some more Ruby code, with line numbering starting at 10.

The following warning is triggered:

/opt/project/vendor/phpdocumentor/guides-restructured-text/src/RestructuredText/Directives/CodeBlockDirective.php:87
Object of class phpDocumentor\Guides\RestructuredText\Parser\DirectiveOption could not be converted to int

[linawolf]

@brotkrueml this PR should solve this: #685