Make it compatible with PHP 7 and WordPress 5

[yannbug]

Message body:

Hello!
I have been using YD Network-wide options for many years now and it has saved my life more than once! I was wondering if you would update it for current wordpress.....I tried to set it up on a new site and it's triggering errors

Warning: Illegal string offset 'plugin_version' in D:\survey\wp-content\plugins\yd-wpmu-sitewide-options\yd-wpmu-sitewide-options.php on line 110

Fatal error: Uncaught Error: Cannot use string offset as an array in D:\survey\wp-content\plugins\yd-wpmu-sitewide-options\yd-wpmu-sitewide-options.php:111 Stack trace: #0 D:\survey\wp-includes\class-wp-hook.php(287): yd_wpmuso_reset() #1 D:\survey\wp-includes\class-wp-hook.php(311): WP_Hook->apply_filters() #2 D:\survey\wp-includes\plugin.php(478): WP_Hook->do_action() #3 D:\survey\wp-admin\plugins.php(193): do_action() #4 {main} thrown in D:\survey\wp-content\plugins\yd-wpmu-sitewide-options\yd-wpmu-sitewide-options.php on line 111