Compatibility for SilverStripe 3.7 and PHP 7 requirements

[sinan-evanshunt]

SilverStripe switched from using Object to SS_Object in order to avoid conflict with PHP 7 name space of Object.

See https://docs.silverstripe.org/en/3/changelogs/3.7.0/

in HasOnePickerField.php I changed Object to SS_Object

<?php public function __construct(DataObject $childObject, $name, $title = null, SS_Object $currentHasOne, $linkExistingTitle = null, $searchContext = null) { .... }