wp_postmeta hardcode bug

Thank you so much for the plugin! We found a bug in the code that was causing issues for us, as our database schema is a little different than default. On line 113 of class-gs-formntr-services.php, wp_postmeta is hardcoded into the plugin. This was causing the plugin to error and not post new submissions to the Google Sheet. I replaced with $table_name which was earlier defined on line 95, and the issue was fixed.