Investigate using v4.0 plugin-update-checker for theme update functionality

Updates to Yahnis Elsts’s [`plugin-update-checker`](https://github.com/YahnisElsts/plugin-update-checker/releases/latest) add theme update functionality. May well be more fully featured than [the current theme updater implemented](https://github.com/HGNM/hgnm-2014/blob/master/theme-update-checker.php) internally to `hgnm-2014` (and based on older code written by Elsts).