Change all uses of $_REQUEST in the SL Updater to $_GET.

http://stackoverflow.com/questions/2142497/whats-wrong-with-using-request

While I haven't run into an issue with the updater particularly, I came across this while fixing issues on a PHP 5.2 site where `$_COOKIE` data was included in `$_REQUEST` for a different plugin.

Since `view_plugin_changelog` is never handled via POST, `$_GET` should be sufficient.