Hmm.. I didn't see any latest release so cloned it instead.
When attempting to enable after dropping in extensions/ I get this error:
2015/08/29 21:07:41 [error] 10335#0: *16093 FastCGI sent in stderr: "PHP message: PHP Notice: Undefined variable: class in /var/www/flarum/vendor/flarum/core/src/Support/ExtensionManager.php on line 130
PHP message: PHP Fatal error: Call to a member function install() on a non-object in /var/www/flarum/vendor/flarum/core/src/Support/ExtensionManager.php on line 58" while reading response header from upstream, client: xxx.xxx.xx.xx server: www.foobar.org, request: "PATCH /api/extensions/s9e-mediaembed HTTP/1.1", upstream: "fastcgi://unix:/var/run/php5-fpm.sock:", host: "www.foobar.org", referrer: "http://www.foobar.org/admin"